Not Blood Siblings

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"He and I were raised side by side, but he was actually adopted so technically it's OK that I'm also attracted to him."

— Twilight Sparkle on her brother, Friendship is Witchcraft

A common cop-out in anime, manga and soap operas when a series is based on Brother–Sister Incest, is to reveal towards the end, or explicitly state in the opening monologue, that they're Not Blood Siblings. Usually, for all intents and purposes, everyone acts like they are. Alternatively, it can be used to explain why everyone's okay with the relationship. Despite its name, the trope is not exclusive to siblings and sometimes refers to cousins, or uncles/aunts and nephews/nieces who aren't biologically related.

This can also be foreshadowing for such a relationship, as many shows usually won't make a big deal about adopted children unless there's a Changeling Fantasy involved.

Sometimes this is a result of Bowdlerization, Media Watchdogs, and Executive Meddling, in that in the original series they were blood relations. It also comes up in Moe, for fans who still feel a bit too squicky about it. First Girl Wins can sometimes explain the prevalence of this trope.

This trope is often related (but not by blood, of course) to and brought up around the Little Sister Heroine trope and the Sub-Trope Flirty Stepsiblings, which is about stepsiblings who are in their teens when their parents marry.

As for reality, it depends. These tropes tend to gross out anyone who has been raised alongside their stepsiblings. If they met as teens or later, though, it can seem unfair that their parents got together before they did.

In older works, when sex shouldn't even be hinted at in family-friendly stories, main child characters are usually conveniently adopted orphans. That way, the main male and female character don't have to be accounted as their actual parents. This situation does lead to a lot of confusion with readers. A boy and a girl are adopted by a family, therefore appear to be brother and sister, but still have a romance thing going on and jealousy when one of them has success with another love interest. Since the situation isn't always explained, this often leads to disturbing, confusing ideas with the readers.

Therefore, in such cases, the story adaptations usually omitted the siblings relationship and the characters almost never treat each other as Like Brother and Sister, so as not to cause unpleasant associations in more conservative times.

Quite often it's just a case of cultural misunderstanding. In Japanese the words for "big brother" (兄さん — nii-san) and "big sister" (姉さん — nee-san) are very commonly used in casual speech to address young people in general, too, and it's not rare that little kids and teenagers will use them to address older but still young friends (or love interests). However, sometimes clarification is needed if the context is not clear enough about it.

For when "siblings" are by choice, instead of blood, that's Family of Choice. Also, do not forget that the ever-growing number of imouto-titles has practically translated this trope into the category Like You Would Really Do It and Captain Obvious Reveal, so even its well-founded use can very quickly get into Cliché Storm. Not Blood Siblings is to Brother–Sister Incest as Faux Interracial Relationship is to Maligned Mixed Marriage. Not to be confused with Blood Brothers, where unrelated characters make a pact and become like siblings. Also see Wife Husbandry for when this trope is taken to an even squick-ier level. For the inverted version — people who find out they're related after thinking they're not — see Surprise Incest.

Examples:

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Anime & Manga

Heavily zigzagged in Alice in Murderland. Stella has been infatuated with Zeno since before they were both adopted into the Kuonji family as children, because he shielded her from debris in the plane crash that killed her parents. Her feelings carry over well into adolescence, when she eventually discovers that her beloved Zeno-nii isn't the same Zeno who protected her, but a shapeshifter who took his place when the real Zeno died in the brutal pre-adoption selection process. Then it turns out the real Zeno was rescued and raised in secret by the family butler, Kokonoe, under matriarch Olga's nose, and both he and his impostor who grew up with Stella have feelings for her. Then it turns out Zeno was never real, but a more confident, outgoing Split Personality of the real child, Tsukito, who was originally too timid to meet Stella without using Zeno as a front and didn't tell her the truth to spare her feelings after meeting her for real. Stella is unable to choose between Tsukito and the fake Zeno, and Olga finally reveals that Tsukito is her biological child with Kokonoe, making him Stella's sibling as well. However, the Distinguishing Mark that should prove Tsukito's parentage isn't found on him, but on Stella, leading Kokonoe to reveal that Olga unknowingly gave birth to twins. While this would implicate Tsukito, Kokonoe goes on to reveal that the twins were Separated at Birth and Stella's twin brother eventually died of illness, while his heart was transplanted into Tsukito.
- The Kuonji family is forbidden to procreate, instead surviving by adopting several orphans at once and raising them as siblings, eventually forcing them to compete in a Battle Royale Game with the last surviving child taking over as head of the family and repeating the cycle in time. The new head is granted the opportunity to revive one sibling, who will more often than not become their spouse and rule the next generation by their side. Olga is revealed to be the particularly brutal winner of her generation who chose to revive her favourite little brother (who was terrified of her well before the battle royale) and marry him against his will.
Invoked in Angel Sanctuary, where the protagonist Setsuna goes and gets a blood test to see if he's related to his sister Sara, whom he lusts after, just for the possibility of this trope occurring. They're 100% blood-siblings, but it doesn't stop them from getting together.
In the manga Ani Com by Yuu Yabuuchi (writer of Naisho No Tsubomi and other Shōjo manga), this is the basis of the plot. It's played out that the remarrying parents expected their two children to get together and get married, and they do.
Hiro and Karada in Living for the Day After Tomorrow; not taken to a romantic end, but still important to the plot. It's implied in the manga (but left unmentioned in the anime) that while not siblings, they ARE blood-related, but Karada never knows (she would be the daughter of Hiro and his aunt conceived just before he went to study in the US).
Eren and Mikasa in Attack on Titan is more Justified than some examples, since his family took her in when she was already nine. While Eren doesn't seem to have any romantic feelings towards Mikasa, Mikasa loves Eren to the point that she's willing to die for him, and Everyone Can See It.
Binetsu Kuukan features a classic example of the trope with new teen step siblings Amane and Naoya. They almost immediately fall for each other, and their respective friends and passersby already think that there's something romantic going on between them. The only barrier between them is their new living arrangements.
Kai and Saya from Blood+, where it is pretty obvious from early on in the series that Kai would hit that like the fist of an angry god. Nobody seems to find this at all strange. When one considers that she was only "adopted" a couple of years ago (ie after Kai had hit puberty) and looked like a teen girl already, it's not so strange.
Interesting variation in the manga Boku no Futatsu no Tsubasa when Hiromi gratuitously reminds Makoto (after a lot of borderline incestuous behavior) that they aren't blood cousins, it foreshadows the reveal that Hiromi's adoptive mother is Makoto's biological mother, meaning they aren't blood sisters, either.
Bunny Drop:
- Defied, as one of the reasons why Yukari and Daikichi don't end up together is to avoid this trope. Daikichi's adopted daughter and Yukari's son have romantic feelings for each other post-timeskip thus it would create trouble if Yukari and Daikichi were ever married.
- Near the end of the manga the characters learn that Rin is not the biological daughter of Daikichi's grandfather after all. This causes Rin and Daikichi to become the Official Couple despite their over twenty-four year age gap and Daikichi raising Rin.
In A Certain Magical Index, Motoharu is in love with his adopted sister Maika. She's the only person he truly cares for.
Cherry Juice features a boy and girl of the same age becoming step-siblings as young children, who eventually grow to have the hots for each other as teenagers. Their relationship is supported by both the girl's friends and her grandmother, who does everything she can to get the two together.
Citrus revolves around two stepsisters who fall in love. A good deal of the conflict is due to them trying to be familial but also being attracted to each other.
Code Geass played with this in one of its many Sound Episodes. Shirley goes through a series of Imagine Spots in which her attempts to ask Lelouch out go spectacularly wrong. In one of the imaginary sequences, his ill little sister Nunnally enters the scene and says that they'll be late for the wedding; as Shirley recoils in shock, the pair claim that they're Not Blood Siblings, so it's okay. In reality, Nunnally and Lelouch are blood-related, though they still have Incest Subtext nevertheless.
Jeremy and Ian of A Cruel God Reigns fall into this category. They became stepbrothers when Jeremy's mother married Ian's father and begin a relationship of sorts after their parents' death.
In Date A Live, Kotori has a crush on her older foster brother Shidou. He only sees her as a sister, which ticks her off. He is willing to kiss her, but only because it is the way to seal her out of control powers. When facing his biological sister Mana, Kotori actually used this trope to an advantage: since they're not blood sibling, Kotori can marry Shidou, Mana can't.
The main plot of The Devil Does Exist; Takeru and Kayano's parents are engaged when they begin their relationship, so understandably ~~they~~ Kayano goes to great lengths to keep it secret. Some people in-universe accept it, and others don't.
Domestic Girlfriend: Natsuo has a crush on his teacher Hina, and in an effort to forget about her goes to a mixer where he has sex with Rui. Later that week, he finds out that his father has remarried, and his new wife has brought her daughters Hina and Rui with her. Natsuo ends up dating Hina, and while the main problem is that he is her student, being step-siblings is also a problem. After they break up and Natsuo eventually starts dating Rui, the problems are downplayed, but still present. Pretty much everyone knows they're dating, but they're still hiding it from their parents.
Dr. STONE has a particularly absurd example: At the end of Part 1, protagonist Senku ends up engaged to Ishigami Village's high priestess Ruri, who then reveals that the village is descended from the only six survivors of the "Stone Beam"—including Senku's own father Byakuya Ishigami, with Ruri implied to be a descendant of him and American pop singer Lilian Weinberg. Shortly thereafter Senku reveals that Byakuya isn't his birth father, which some readers felt was unnecessary since over 3700 years have passed since then, and Senku wasn't interested in her (or any other villager) in the first place.
In Family Compo Masahiko has feelings for Shion, who acts as his sister and is referred to as so. But while Shion isn't his sister, she is his cousin.
In Fate/kaleid liner PRISMA☆ILLYA, Illya has a crush on her adopted brother Shirou, even at one point kissing him in a dream. Shirou is completely oblivious to any girl's feelings, including hers.
At one point in Fire Tripper, Suzuko falsely believes that she and Shukumaru are brother and sister by blood. When she discovers that the two of them actually aren't blood-related, she happily returns his affections and marries him.
Kurei and Kurenai from Flame of Recca. Mori Kouran was already the adopted father of Kurei, then he was revealed to also have adopted Kurenai when she was about to be killed. Let's not forget that both Kurei and Kurenai were in love with each other and actually a couple in canon.
In the manga and anime of Georgie!, two of Plucky Girl Georgie's love interests are her brothers, Abel and Arthur (The other being a nobleman's son Lowell Grey). In the anime, aside from some mildly questionable scenes, Georgie ends up with none of them. In the manga, however, Georgie has sex with Abel, then has his child after his death, then returns to Australia where Arthur is waiting for her.
Tetsuya and Jun from Great Mazinger were two orphans taken in by Kenzo Kabuto and were raised together. Technically they are adoptive siblings, but they are not related by blood at all, so no one bats one eyelash regarding their ongoing Belligerent Sexual Tension.
In the anime Happy Lesson, Chitose's sisters aren't really related to him and are clearly invoking a Big Sister Instinct and Cool Big Sis, but this isn't milked for UST at all.
Hetalia: Axis Powers: In-universe, it's rather unclear how countries are "born" and form physical "relations" with one another, but a few sibling pairs have been confirmed to be unrelated by blood, including:
- Switzerland and Liechtenstein.
- China and Japan; notable in that the status of China's other siblings^note are ambiguous as to if they're related to China by blood.
- America and England is a special (and odd) case: When America was a child, England adopted him as a younger brother, but during the American Revolution, the teenage America declares that he's "not [England's] little brother anymore."
- Averted with Norway and Iceland—the latter's "DNA testing results" (archaeological excavation) prove that they are, in fact, related.
In Hot Gimmick, Hatsumi is unaware for a large portion of the series that her brother Shinogu is adopted. Shinogu, however, is aware and even moves out of their family's apartment as part of a plan to make Hatsumi see him as more than a brother. His compassionate nature also arguably makes him a preferable match for Hatsumi, compared to her Jerkass boyfriend Ryoki.
- The novel has a Shinogu/Hatsumi ending, for those of us who thought the manga's ending was TEH FAIL.
Subverted in I Don’t Like You at All, Big Brother!!. Shuusuke is determined to treat Nao like a sister and nothing else, despite him totally having the hots for her. Nao, on the other hand, is actually slightly disappointed to find out that they're Not Blood Siblings, because it makes their attraction to each other less taboo.^note
The Irregular at Magic High School plays with this trope when it turns out that Miyuki is an improved magician, i.e., she was artificially created and then genetically modified to have powerful magic powers so that she could stop her brother from destroying the world, if needed. In this way, although she and Tatsuya are certainly biological siblings and have the same parents, their genes are different enough so that their potential Brother–Sister Incest connection is safe for their offspring. Later they are engaged under the guise of cousins, although their feelings are not mutual.
Played with in the light novel series Isekai Konyoku, with the hero being summoned to another world in his own body and later discovering that his biological sister had been resurrected into this world in a new body, resulting, as he puts it, in her being his blood-related-yet-not-blood-related sister.
In the Okazaki Tsuguo manga Justy the titular character has two of this sort of sisters (the older one "adopted" him when she found him as a lone child, and then he "adopts" the younger after he kills her criminal ESPer father), and they both have crushes on him. Justy ends up romantically involved with the older one, Jerna. And he continues calling her "neesan," even when they're nude and postcoital in bed together.
Parodied in the Kaguya-sama: Love Is War bonus chapter "Kagukoku: Miyuki and His Girls", where Shirogane wakes up to find himself in a generic harem series. Naturally, his little sister Kei is cast as the step-sister with a brother complex who lives alone with him while their parents work overseas.
Shirogane: Yeah that's definitely a red flag. Why hasn't anyone called child protective services?
KissXsis runs on this trope. Keita's twin stepsisters compete with each other for his affections, much to his dismay. His parents not only are aware of this, but actively encourage him to go for one of the girls, constantly reminding him that "it's not by blood, so it's OK!"
In the OVA adaptation of the bishoujo game Ko-ko-ro..., Souji is shown in flashback to be adopted by the Kuonji family. This is apparently not lost on his younger sister, Asuka; though she addresses Souji as "Aniki" (Japanese for "older brother"), she regularly initiates intercourse with him and secretly watches him having sex with other girls in his bedroom.
- When in public, Asuka is openly affectionate and quite clingy with Souji, acting much more like his girlfriend than his sister, and seems to relish the shocked stares of people who see them together.
- The OVA scenario is about as shocking as in the revised version of the game, in which Souji and Asuka were merely stepsiblings, but neither is as shocking as that of the original release of the game, where they actually were blood siblings who had sex with each other and were also sexually abused by their parents.
Completely inverted in Koi Kaze, where Koshiro and Nanoka develop an attraction for each other and then find out they are blood siblings who haven't seen each other in 10 years. The story revolves around how they cope—or not—with the fact that they can't get over their feelings for each other. In fact, Koi Kaze uses the anecdotally-supported phenomenon Genetic Sexual Attraction as a factor in Koshiro and Nanoka's interactions.
Manga scenarist Shirodaira Kyou apparently loves playing with this.
- Played straight in Blast of Tempest with Mahiro's dead sister.
- A variation in The Record of a Fallen Vampire: Akabara's adopted daughter fell in love with him, though he still sees her as his little girl. As a bonus, Akabara later married her half-sister.
- In Spiral, Ayumu is strongly hinted in the anime and confirmed in the manga to be in love with his sister-in-law. Inverted with Kousuke and Ryouko, who have blatant Ship Tease much before we learn (in the manga only) that they're actually half-siblings—but they've always known it, and though they've been raised apart, it has always been an obstacle to their relationship. (They eventually do make it to Official Couple status.)
In Listen to me, girls. I am your father!, college-age Yuuta is forced to raise his three young nieces after his sister's disappearance. The older two sisters are actually his sister's stepdaughters, while three-year-old Hina is the only one related to Yuuta by blood. The oldest sister, Sora is fourteen and immediately develops a crush on Yuuta. At the end of the light novels, Yuuta and Sora get married, coupling this trope with Wife Husbandry.
Love Hina introduced the hitherto unmentioned Kanako, Keitaro's quasi-adopted sister. Ironically Keitaro is the only character who never rationalizes Kanako as being 'fair game', while Naru ends up taking his role of clumsy awkward relationship handler for a period of time because she does.
In Magical Girl Lyrical Nanoha StrikerS the hinted-at Toy Ship of Erio and Caro may sort of fit this in that Fate ends up being the legal guardian of both (though Erio is technically under the custody of Fate's adoptive mother Lindy due to her having been too young to put her name on the paperwork). The accompanying StrikerS manga has them repeatedly state that they are technically adopted siblings... while lathering on even more Ship Tease. Unlike other non-blood siblings, they had never met prior to StrikerS, and had no pre-existing sibling relationships (they even begin on a Last-Name Basis).
In Marmalade Boy, Miki and Yuu's respective parents swap marriage partners, thus making their children step siblings. The kids fall in love with each other, and though they worry what their parents will think they don't see it as an insurmountable problem. Drama ensues when Yuu discovers some circumstantial evidence suggesting that they are blood siblings, and quickly breaks up with Miki, gets the hell out of town, and leaves them both to suffer. They finally conclude that they still love each other and want to get married, but their parents reveal that it was just a misunderstanding and they're not blood siblings after all, making it a Double Subversion.
The manga, Me & My Brothers, takes great pains to remind the reader nearly every chapter that the heroine is in fact not blood-related to the titular brothers, despite the also-incessant reminders that they are all family and that Sakura is their "little sister." This is used quite blatantly so Sakura can be obsessively fawned over by her new guardians — and eventually fall in love with one of them — without the Squick.
Pristine example in Miyuki: The main character soon discovers that his stepsister Miyuki is not blood-related to him. She ignores it until the end of the manga when he proposes in the middle of her wedding to one of his best friends. She then breaks up the wedding, and they eventually get married.
In NAKAIMO - My Little Sister is Among Them!, Kumagoro Mikadono dies and leaves his fortune to his son Shougo, but only if Shougo goes to a certain school to meet a suitable wife. The very first day, a girl speaks to Shougo through a high window, claiming to be his biological sister and that she's going to marry him, but she refuses to give her name. Shougo, not wanting to marry a blood sibling, thus has to find out which go the many girls clamoring over him is the one who spoke to him through the window. The original Light Novel series finally reveals that Konoe, the girl who'd spoken to him, is indeed Kumagoro's biological daughter, but Shougo is not his biological son, and they end up together. The anime, on the other hand, ended before the novels did, leaving things up in the air.
Has the heck spoofed out of it in Ninja Nonsense, where one of the main characters, a yellow Pac-Man-esque anthropomorphic football who is really a Ninja Elder refers to this particular trope and, being a pervert, has a particular high esteem. "If the Master says there is a cute little non-blood related sister, there IS a non-blood related sister."
Oni Ai: Akito and Akiko are apparent twins, and the entire show chronicles Akiko's efforts to get in the pants of her older brother. Only in the last episode is it revealed that Akiko's been, for the sake of his sister, keeping his adoption a secret all along.
Penguindrum has two instances of these:
- In Episode 19, it is been revealed that Himari is not Kanba and Shouma's blood sister; she is actually a girl rescued from the child broiler and was adopted into Kanba and Shouma's family, and she is in love with Shouma.
- In the next episode, this is also revealed Kanba is not really Shouma's blood brother as well.
- Then provides a surprise aversion: Kanba and Masako are Half-Identical Twins. Guess Masako is into half-twincest?
Silver and Blue from Pokémon Adventures were both kidnapped at a young age and were raised as siblings. The manga has implications that Silver might have a crush on Blue.
The animated adaptation of Pop Team Epic features a parody of such stories, titled Don't Call Me 'Sister'! (Or IMO YOBA for short). Heroine Iyo Sakuragi and her popular crush Hojo are suddenly made into siblings due to their parents (Pipimi and Popuko) remarrying each other, and the story follows some typical plot beats of the pair trying and failing to be a regular family. Their problem, however, lies not with each other, but their parents; eventually Iyo gets so fed up with them that the story takes a turn for the violent.
None of the Elmore kids in Psyren are blood-related. Probably why Fredrica and Marie is one of the most common pairings.
In R.O.D the TV Michelle, Maggie, and Anita are "sisters in law" rather than actual sisters, though this is more for plot reasons than pairing them together.
Inverted in Revolutionary Girl Utena: when Nanami thinks that she and Touga aren't blood siblings, her world collapses since she's very fixated on Touga and pretty much all of her self-worth hinges on her status as his sister. Turns out Touga is her brother by blood, as they were both adopted into the Kiryuu family but Nanami was too young to remember.
In Recently, My Sister Is Unusual, the ghost Hiyori Kotobuki often possesses Mitsuki Kanzaki and uses her body to hit on Mitsuki's stepbrother Yuya. Mitsuki finds this disgusting and humiliating, constantly managing to force Hiyori out of her body. However, as time goes on, she starts fantasizing about and blushing around Yuya even when Hiyori is not there, and getting jealous of any girls that get close to him. Yuya is confused about the whole thing, thinking Mitsuki has a split personality.
Sensual Phrase has Atsuro in a relationship with his sister Yuka. Their exposed relationship even causes a scandal, but Atsuro casually reveals to the press that they are stepsiblings.
Played for Laughs in Servant × Service. When Taishi mentioned he actually has a girlfriend, Hasebe immediately suspected this between he and his Tsundere Brocon sister Touko. It is in fact averted, of course, as his girlfriend is Chihaya.
In Six Half, main character Shiori is worried over developing feelings for her brother Akio, whom she doesn't fully see as a brother due to developing amnesia from a motorcycle accident. As Akio reveals to his girlfriend, Shiori isn't related to him at all and, according to Akio, she knew this before the accident, too.
Pointed out in the visual novel-based OVA _summer, where Osamu, the typical idiot best friend character, points out "Sana's technically your stepsister, so you could go for her if you want..." when looking over the many girls the protagonist has gathered around him.
In Sword Art Online, Suguha has had a strong but secret crush on her older brother Kazuto (better known by his screen name Kirito). It's mentioned that Suguha's parents are actually Kazuto's aunt and uncle, who adopted him into their family when his parents died in an accident, meaning that he and Suguha are actually first cousins rather than true blood siblings. However, it's pointed out that Suguha knows that since they were raised together, the rest of her family would oppose them getting together, which is why she tries to get over Kazuto.
It's subtly hinted and discussed in Tantei Gakuen Q about the relationship nature of two stepsiblings Tomoe and Ukon Kiriyu in Setsugekka Murder Case. It's apparently one-sided since the side shown to readers is Tomoe's who tries her hardest to prove Ukon's innocence before The Reveal (he was framed as murderer of his other two stepsisters) while Ukon himself pretends to be an airhead as he is suspicious that all his stepsisters and stepmother were involved in his father's past accident. Kazuma assumes the trope is in play and the stepsiblings' father Samon asks what Tomoe thinks of Ukon. Too bad she is the true murderer. While later she admits her feelings for Ukon are real, when Nanami disguising himself as Ukon confronts her about the murder she immediately stabs him.
Played with in Chapter 104 of the To Love Ru manga with Rito's younger sister, Mikan "revealing" to Rito that she's not related to him by blood in an unexpected moment of sweetness. Of course, she was just doing it to mess with her older brother... or was she?.
- And then revisited with a megaton-sized blast of Ship Tease in the entirety of Chapter 157.
Vampire Game has Laphiji and Seilez, although, as Ishtar has said, the family's so inbred everyone's practically married to their siblings anyways.
In World Break: Aria of Curse for a Holy Swordsman, though Ranjou and Moroha were blood-related in a past life, in their current lives they aren't. However, Ranjou plays the trope up as if they were blood siblings, and flies into a rage when Shizuno gets a little too friendly with Moroha.
Yoake Mae Yori Ruriiro Na has two sisters for the lead, neither of whom is his blood sister, and both of whom are haremettes in the game. (For the record, Sayaka is his cousin, and Mai was adopted).
It's eventually revealed in Yuureitou that Satoko and her Abusive Parent Marube are most likely not blood-related. Marube had been molesting her and when Satoko learned the truth of their bond she became furious and ran away. Marube apparently has no qualms with blood incest though, as he later tries to have sex with his biological son, Tetsuo.

Comic Books

In an early Alpha Flight story, it's strongly implied that Marrina's adopted brother was in love with her.
Batman:
- Downplayed, but it does show up twice in the case of the second Batgirl, Cassandra Cain, who is the only sister among Batman's adoptive children before Flashpoint (DC Comics).
- It is implied that the fifth Robin, Damian Wayne, had a Precocious Crush on Cassandra, but it is also possible to read the apparent infatuation as something close to hero worship, given that Damian was raised by a secret society that saw her as its savior and didn't meet her until he was ten.
- Cassandra was shown to have a romantic interest in Tim Drake when she was controlled by Deathstroke in Teen Titans, which caused quite a bit of backlash, since they had been Like Brother and Sister even before either of them were adopted by Batman.
- Back in the early 1980s, DCU stories set on Earth-Two hinted at the mutual attraction between the Huntress (Helena Wayne, Earth-Two Batman's daughter) and the local version of Dick Grayson. It pretty much remained subtext, with both parties insisting all too earnestly that even though they weren't biologically related, their relationship would be too wrong to even contemplate. This was essentially defied in the Justice Society of America story "Thy Kingdom Come", which featured a return visit to a new version of Earth-2. In this reality, Helena realised that she was in love with Dick Grayson after her then-boyfriend DA Harvey Sims was attacked by the Joker, the aging criminal attempting to "recreate" Batman's deceased Rogue's Gallery. Given the circumstances of Sims' disfigurement, Helena feels that she cannot leave him as she never had the chance to say "No" to his proposal, but he has been left crippled and confined to hospital so they can never have a true relationship.
In ElfQuest, it is hinted at in the comic, and expressly stated by the Word of God, that Cutter and Skywise have had sexual relationships. It is also mentioned in the comic that they are "brothers in all but blood." Of course, the elves are said to not have the same concept of relationship as (modern Western) humans do, even though nearly all of them are paired in "life bonds" (i.e., marriages).
Mister Immortal from Great Lakes Avengers learned he was immortal after his foster sister's death, which became Narmtastic after his repeated botched suicides.
In The Kingdom (DC Comics), Nightstar, the daughter of Nightwing and Starfire, hooks up with Ibn al Xu'ffasch, the son of Batman and Talia al Ghul. Nightwing is Batman's adopted son while Ibn al Xu'ffasch is his biological son who wasn't raised by him. When Ibn al Xu'ffasch and Nightstar, fall in love, even though they're technically uncle and niece, this is part of why Nightwing isn't especially happy with their relationship.
In Prince of Persia: The Graphic Novel, Layth and Guilan are married before the story begins. They were raised as brother and sister after Saman, Guilan's father, defeated the Caliph's army and killed Layth's real father in battle.
In Runaways, Molly's dad is her mom's foster brother.
The Sandman (1989): In the seventh volume, it is heavily implied that Despair II has an unrequited crush on her adoptive brother Destructioncomic.
Before Suske en Wiske featured both titular characters, it was Rikki en Wiske, starring Rikki, his younger sister Wiske, and their aunt Sidonia, who raises them. It was found that Wiske was the strongest character of the cast and to give her more emphasis Rikki was replaced by Suske, an orphan Wiske and Sidonia met on their next adventure and who joined their family. Very, very, very few comics handle differences in their lineages and for any casual reader, Suske and Wiske appear siblings by blood. This caused a lot of disturbing scenes where Wiske is jealous of Suske's success with other girls and a case of Brother–Sister Incest appeared to be going on.
Happens not once but twice in The Umbrella Academy; first between the Rumor and Spaceboy, and then the Kracken's obvious unrequited feelings for Vanya, as well.
In the German comic Wendy, Wendy dates her foster cousin for most of the story until her aunt decides that she doesn't want to adopt him and his twin sister after all, as they aren't as rich anymore. After that, he breaks up with Wendy to date his previous foster sister Vanessa instead, while being implied to still live in her house. It's weird.
In X-Men, Nightcrawler's "true love" is his adoptive sister Amanda "Daytripper" Sefton, even though they were raised together since infancy. To be more specific, Kurt dated a woman named Amanda Sefton, who would later be revealed to be Jimaine Szardos, Kurt's adoptive sister who he was raised alongside as a sibling when Jimaine's parents found Kurt abandoned as a baby. Despite tricking Kurt into believing her to be someone else, Kurt continued his romantic relationship with Jimaine fully acknowledging their sibling relationship. This was portrayed as a bad idea though — not because of their prior relationship, but because Amanda was portrayed as not-quite-right in the head. Nightcrawler (as Mystique's blood son) and Rogue (as her adopted daughter) have potential for this, as well. Which if it happened would be less potentially squicky than Amanda; they never even met until adulthood. He still flirts shamelessly with her quite often though.

Comic Strips

Dick Tracy: Silver and Sprocket Nitrate, a pair of movie-themed Con Man villains, were introduced as brother and sister and seemed to be close in some freaky ways. The police later found that they were two orphans adopted by another pair of cons, and not related by blood.
Nero: Petoetje and Petatje are also adopted and raised as brother and sister by Madam Pheip.

Fan Works

Few 101 Dalmatian Street fanfics often use this excuse to pair up Dolly and Dylan together, they are confirmed to be stepsiblings by the original creators but there is nothing suggesting it in the show itself.
Phineas and Ferb often fall into this; they're canonically stepbrothers, but refer to each other as brothers and have been raised together since they were toddlers. Slightly less commonly they'll be paired with their older sister (biological for Phineas, step- for Ferb) Candace.
Drake & Josh has a few examples between the titular stepbrothers.
Invader Zim: There are not very many Dib/Gaz shippers, but the few fics you'll find of it tend to use the Word of Saint Paul concept of Dib being an Artificial Human to justify it. A particularly popular idea is that Dib is a clone of Membrane and Gaz is a clone of his unseen, presumably-dead wife.
Life with Derek:
- Derek and Casey is a non-canon ship that's largely popular with the fanbase, as opposed to all the other love interests that are introduced throughout the series. The fact that half of the show focuses on the interactions and the relationship between the two stepsiblings doesn't help.
- As a sort of Beta Fan-Preferred Couple of sorts, there's also Edwin and Lizzie, the younger siblings of Derek and Casey respectively. Doesn't help that both sometimes use the closet as a meeting place, which immediately has shippers thinking of the Ten Minutes in the Closet trope.
Many The Chronicles of Narnia fans use this excuse in Edmund/Lucy and Peter/Susan stories.
The Twilight fanfiction Dark Whispers features Bella and Jasper as the main characters. They are brother and sister, their father is abusive, and Jasper ends up killing him before he runs away with Bella. The main focus of the story is how an almost obsessive Jasper engages in all kinds of sexual activities with his little sister before they find out at the end that they're not actually related.
Twilight fic "Bella Hale" sees Alice taking steps to avoid this as much as possible after the Cullens adopt a six-year-old Bella following her parents' deaths. As soon as Alice sees a vision of Bella and Edward together in the future, she takes subtle steps so that Bella is listed on paper as Rosalie and Jasper's direct sibling, thus creating a sense of distance so that her future relationship with Edward won't seem quite as weird.
Even though BIONICLE is supposed to be a No Hugging, No Kissing universe, (any romantic subtexts in the movies are discontinuities) there's a good deal of romance in Fan Fiction. A lot of it is between members of the same team who call each other "brother" and "sister". However, it becomes much less squicky when you realize that the entire cast is made of mostly machine cyborgs and that "brother" is more a title of endearment than anything.
In the Gender Flip-based John Lane fanfic series, this happens in a more justified situation; after Helen learns how totally neglectful John's parents are, she takes them to court and takes custody of John Lane, who at that point is already in a budding relationship with Daria.
Mari tries to use this argument with Uri in Nobody Dies; he shoots her down regardless, still squicked by the prospect.
In one Naruto fan comic, Temari discovers her father's secret papers, which prove that she was actually adopted, and doesn't have to hide her feelings for her younger brother Gaara any longer. Kankuro then wonders if he can do the same for Gaara, but Temari beats him up while reminding him that he and Gaara are still related, since she's the one who was adopted.
Oneiroi Series: Terentius briefly tries to use this rationale to excuse himself from wanting his adopted half-sister, but he quickly breaks down, admitting to himself that it really doesn't matter and that he was lusting after his little sister.
Subverted in Relationships Series. When it comes to light that Nove and Ginga, who are in a three-way relationship with each other and Wendi, are genetically related, this revelation motivates Genya's decision to send Ginga on a mission to separate her from Nove and Wendi.
In legolas by laura, the eponymous elf adopts the heroine Laura as either his sister or his daughter (it's not really made clear). After rescuing her from orcs, he agrees to "be your boyfriend," even though she's his adopted sister/daughter... and ten years old.
The Star Wars fanfic "Father and Son" has Han and Leia as a couple (as in canon) despite the fact that her (and Luke's) birth parents adopted and raised Han for several years before the twins were born. Somehow, nobody (not even Vader or Padme's extended family on Naboo) sees anything wrong with the two being a couple. (Admittedly, all three siblings were sent to separate planets after Anakin's Face–Heel Turn and Padme's Death by Childbirth and didn't find out that they were siblings until after they were all adults. But still.)
Most fanfiction for the pairing of Shadow the Hedgehog and Maria Robotnik from Sonic The Hedgehog downplay the Like Brother and Sister aspect their relationship may or may not have had. The fact that they're not even the same species certainly lends weight to the argument against any romantic relationship being incestuous... Although for some people that might just create another reason to be against it.
Thor and Loki of the Marvel universe tend to be the subject of some arguments regarding whether or not pairing them constitutes incest. On the one hand, they were raised together and usually refer to each other as brothers (albeit somewhat sarcastically), but being that Loki is an adopted frost giant and goes by Laufeyson and not Odinson, whether or not they see each other as siblings depends on the writer.
A few Yin Yang Yo! fans use this to justify the Twincest.
A source of some discomfort to Shinji in Doing It Right This Time. First he had what he thought was a near-miss with Surprise Incest before the whole Peggy Sue thing happened, but now it turns out Rei wasn't a clone of who everyone thought she was and they're definitely not even siblings by adoption (Rei's pretty emphatic about that) and he has no idea if that makes the time he accidentally saw her naked better or worse. Especially because he has a bit of a thing for Asuka as well.
As the quote above demonstrates, Friendship is Witchcraft's Twilight Sparkle justifies having a crush on her brother Francis (that universe's version of Shining Armor) by pointing out that Francis was a Doorstop Baby left at Twilight's house. Even after her explanation, the other ponies still find the attraction creepy. Twilight wins in the end, and eventually marries Francis.
Invoked in the Frozen fanfic We Can't Be Wrong Tonight. Elsa and Anna order fake documents created that show that Anna was adopted. The two are biologically sisters, however, they want to be open of their relationship and marry each other. Anna has a resemblance to her father but looks different enough from the rest of the family for it to work.
Quite a bit of Teenage Mutant Ninja Turtles fans who like "Tcest" write the brothers as adopted siblings. It's vague in the various continuities if they're blood-related or not.
In the Frozen fanfic Becoming Family, Elsa's adopted daughter and Anna's biological son fall in love. The cousin incest element never gets referenced.
Played for Laughs in the Community fanfic If It's Real (It Stains Your Hands Like Wine). During Abed's crappy movie, Jeff and Annie almost kiss until Britta reminds them that their characters are supposed to be father and daughter. Abed declares that the movie does need a kiss, however, so they'll just say that Annie is someone else's daughter who was kidnapped as a baby and raised by Jeff. Britta points out that this is worse, but everyone else thinks it's fine. Abed eventually decides they don't have time to film the kiss after all, and it's implied that the only reason he suggested it in the first place was to push Jeff closer to Annie.
In Reign of Marco, Star was adopted as a child by the Diaz family. She takes full advantage of this in the present.
In Their Bond, Link was adopted by Princess Zelda's father as a child and the two were raised as siblings. As teens, Impa catches the two being intimate but it's a bit different than it initially seems. Yes, it was sexual, but neither actually wanted it. They're not even attracted to the opposite sex. Zelda uses making out with Link as a way of dealing with her past sexual abuse by Ganondorf.
re:Bound (RWBY) is a RWBY fic which ships Ruby and Neo. It's revealed several chapters in that Neo is the adopted daughter of Raven, taken in after Raven's tribe killed six-year-old Neo's parents. Raven is the biological, absentee mother of Ruby's paternal half-sister Yang. So they're both half-sisters to Yang in a sense, but on different sides of the family tree.
Fate/Harem Antics: Shirou is the adopted son of Illya's father. While they didn't grow up together, and Shirou only found out she existed when the War started, Illya still refers to him as "big brother" (despite actually being older than him). She ends up as the third person in Shirou's harem, after she kidnaps him and takes him to her castle for some "alone time."
Similiar to the Thor and Loki example above, there's room for debate as to whether shipping John and Arthur in Red Dead Redemption II counts as incest. We know John was taken in by the gang when he was twelve, so he was at least a preteen when him and Arthur met (and thus already past several formative years), but that doesn't change the fact Arthur played the role of a big brother for a good amount of time. They refer to each other in-game as brothers a few times as well, though often begrudingly. Most fanworks make it very clear the two didn't develop feelings for each other until John was well of age, at least- and others simply put them into an alternate universe where they didn't meet until they were both adults. Also worth taking into account is the fact that Dutch's gang was anything but conventional in terms of societal values and taboos.
Undertale:
- Fan works pairing up Asriel and the first human, which would also be a case of Interspecies Romance. We learn at New Home that the human was essentially adopted by his parents and that Asriel and the human "became like siblings" over time. When we see evidence of just how devoted he was to them though, it can be read as a case of Puppy Love by some.
- Fan works pairing up Frisk and Asriel could fall into this, since Asriel's parents try to take care of the Frisk and offer to adopt them as well, and accepting the offer is the most popular ending with fans. Fans who want to try to avert this trope can Hand Wave it by simply setting their work in an ending where Frisk chose not to stay with Asriel's family.
- Like the above example, fan works pairing up Frisk and Chara arguably also qualify, since Frisk can be adopted by the same parents who'd adopted Chara, and can be downplayed in the same way.
Children of Remnant: Blake and Pyrrha are Jaune's adopted sisters, but they are both obsessively in love with him. Pyrrha channels this obsession into devotion and service, but it's still obvious she wants to be something more. Blake, surprisingly, is romantically but not sexually in love with him; her preferred endgame is that Jaune and Pyrrha get married so that Pyrrha can handle his physical needs while Blake handles his emotional needs. Jaune is mostly oblivious.
Once Upon a December Night: While this version of April and Donatello were raised as brother and sister, both Word of God and subtext confirm that this is a "Apritello" fic.
No Matter What, an Amphibia story part of the Fourth Light Series, features a discussed/justified case. Marcy gets adopted by Anne's parents after hers are arrested for threatening her life: Anne and Marcy were dating before this, but they don't have a problem with it since they're not related by blood and can legally get married one day so long as they prove it, a true fact in the USA.
Frozen Wight: In-universe, Played for Laughs. Elsa and Anna are extremely annoyed to find that an artist has been drawing and writing incestuous stories about them. He insists that they're not incestuous; they take place in an alternate universe where they're not actually sisters. This excuse doesn't save him from the defamation charges.

Films — Animated

The Lion King:
- We see Nala's mom, but there are constant debates on who Nala's father might be, and thus to what extent she is related to Simba. Scar and Mufasa are the only male lions in the Pride, and—traditionally according to real lion behavior—Mufasa would be father of all the cubs. Barring this, she may be the daughter of Scar, which is uncomfortable for several other reasons. In some drafts (and in the Broadway musical), Scar actually tries to make Nala his consort, which is the real reason she exiled herself from the Pride. You see what the problem with that is. Moreover, in the sequel, Nala and Simba's daughter Kiara hooks up with Scar's adopted son (well... supposedly, see the following bullet-point), which is just as weird. According to The Lion Guard, Nala's father is an unknown male who died or disappeared in unknown circumstances prior to the events of the films. Some fans have been reluctant to accept this explanation (as well as other retcons done by the series) and declare it Fanon Discontinuity altogether.
  - Nala's paternity aside, her maternity also raises questions. All the female lions in a pride are related—mothers, daughters, sisters, aunts, nieces, etc. So the paternal line notwithstanding, Simba and Nala's mothers, Sarabi and Sarafina, are kin in their own right, making Simba and Nala related through the maternal line as well (though more distantly).
- In The Lion King II: Simba's Pride, isn't it convenient that Kovu isn't really Scar's son, making it perfectly okay for him to get it on with his cousin (technically, his first-cousin-once-removed)? This was a last-minute change as Kovu was originally Scar's biological son with a Strong Family Resemblance before the writers realized the minor case of incest in point (some dubs still kept them as cousins, though).

Films — Live-Action

The Alvin and the Chipmunks films changed the Chipettes so that they end up being adopted by Dave instead of Miss Miller. This means that they're legally the adopted siblings of the Chipmunks while still being their love interests.
The Spanish movie Los Amantes Del Círculo Polar extends this trope from Flirty Step Siblings in that Otto's divorced father marries Ana's widowed mother first when the two children are just ten years old. Then, as they are living together in the same house, a teen romance gradually develops between Ana and Otto into a full-blown sexual relationship which, however, they manage to keep completely secret from their parents/stepparents to the end. Curiously, the possibility that the relationship between two opposite-sex stepsiblings could ever go beyond familial love never does seem to occur to either of their parents/stepparents or much of anyone else, which is why the two are able to conceal their nightly liaisons from them so easily.
A very clumsy Western example: Ator, the Fighting Eagle (the first Ator film; MST3K fans might be more familiar with its sequel Cave Dwellers). Ator, the titular swordsman, falls in love with a woman whom he thinks is his blood sister; when he asks their father permission to marry her, the father happily reveals that he is in fact, adopted. (Instead of, you know, being totally squicked out by the idea that he would be in love with his blood sister in the first place).
In Big Trouble, Matt Arnold and Jenny Herk's parents meet through each other, have sex, and eventually get married. Meanwhile, their relationship is somewhat implied to be a bit more than platonic.
The Big Wedding: Nuria is Alejandro's biological sister. When she and his adopted brother Jared have a strong attraction for each other, she at one point objects given this on them having sex. He argues however that as they're not blood related there isn't a problem. They later do have sex together and Alejandro just rufely observes this as his siblings getting it on.
Bushwick: This is apparently the case with Lucy and Belinda, as Lucy explains to Stupe that they were both raised by Grandma, meaning they're sisters in that way. As neither looks anything alike (Lucy is white, Belinda most probably a Latina) they likely aren't even half-sisters.
Clueless, where Cher realized at the end of the movie that she had been unconsciously harboring feelings for her former stepbrother. Their parents got married when they were both teenagers and later divorced, so technically they weren't related at all at that point. Still, it's a bit odd given that he still considers Mel his father, even calling him "Dad" in his first scene.
In Cruel Intentions the main character Sebastian is obsessed with his stepsister Kathryn, who uses the promise of sex to get him to help her with her revenge.
Zhivago and Tonya, who end up married, in Doctor Zhivago.
In the 4th movie of the Dollanganger series, Seeds of Yesterday, adoptive siblings Bart and Cindy do fall in love and eventually marry, but the narrative's framing of this is actually a Subverted Trope. After a great deal of UST, when they first hook up, Cindy justifies the relationship by invoking the trope.
Cindy: All my life you made me feel ashamed for being adopted, but now I am so happy I am. I mean, it's not like we're blood sibs like Mom and Dad. [...] It's not sin; not for us; not really.

: However, Bart ignores her words. This reasoning doesn't seem to land with him; he doesn't seem to think it makes a difference. Before Bart can move forward in his relationship with Cindy, he first has to make his peace with incest and accept the relationship between his mother and step-father (who are blood siblings). Only then can he get together with Cindy, knowing that it is incest and being ok with that.

The French movie The Flower Of Evil has a pair of step-siblings who have had the hots for each other for years. Nobody really thinks this is squicky; in fact, some of the family seem to wish they'd just get on with it.
In Heart of Darkness (1958), Maria Kurtz and Charles Marlow were raised together, but they each make a point of emphasizing that they're not really brother and sister. Maria is openly attracted to Marlow; Marlow considers a romance out of the question for other reasons.
In Lone Star State of Mind, Earl and Baby were already dating when his mother and her father decided to get married. Since they're not actually siblings, they don't see anything wrong with their relationship. A few people still harass them about their technical incest.
In La Roue, Elie was low-key attracted to his adopted sister Norma before he found out they weren't biologically related. After he does find out, he realizes that he wanted to hook up with her all along.
In the 1955 French movie Le fils de Caroline chérie, Conchita knows that Juan was adopted at a young age and uses it to rationalise her feelings for him. However, Juan doesn't learn this until a third of the way through, and the youngest sister Pilar doesn't learn this until the end of the film... and they're quick to jump to talking about marriage after this reveal.
The film Mary Shelley's Frankenstein plays this up far more than the novel (see below). This culminates in a rather strange line as Victor and Elizabeth are about to make love on their wedding night.
Elizabeth: Brother and sister no longer...
In Mr. Nobody, Anna and Nemo are schoolmates who become stepsiblings in their mid-teens and fall in love, much to their parents' dismay. Interestingly, Nemo is shown to have three love interests and Anna is depicted as being the right girl for him.
My Girl 2: Vada and Nick share The Big Damn Kiss near the conclusion of the film even though they're gonna soon be cousins and it seems implied that they'll continue to have a romantic interest even after that. That said, the former points out to Nick that they'd be more akin to "strangers who didn't know each other and were suddenly related."
This was brutally parodied in Not Another Teen Movie when their Kathryn expy constantly tries to seduce her brother.
Jake: That's disgusting. You're my sister!
Catherine: Only by blood!
In Relative Fear, Adam's biological parents were foster siblings.
Connie: He could have been my brother. But he didn't want to be my brother. He said I was the prettiest thing he ever saw. I was just a kid then, I didn't know anything! I was thirteen and wide-eyed!
Richie and Margot in The Royal Tenenbaums are a somewhat more realistic treatment, as their relationship is considered inappropriate despite everyone knowing that Margot is adopted. Interestingly, Anderson had originally intended for this to be an aversion, based on a real-life friend who was in love with his own sister. It was changed late in the process when Anderson came up with the Running Gag of Royal introducing her to everybody he knows as "my adopted daughter, Margot Tenenbaum."
Infamously inverted in Star Wars: Return of the Jedi with Luke and Leia, with the revelation that they were the siblings... one movie after Leia kissed Luke on the lips in The Empire Strikes Back. Their actors' reactions tell the whole story. Leia's assertion that "Somehow, I've always known" just makes this worse.
In Teeth, Brad likes his stepsister a little too much. She doesn't know it for most of the time and is horrified when she finds out.
Subverted in the 1991 French Film "Toto le Héros" (Toto the Hero) The main character is convinced as a young boy (about 10) that he was switched in the hospital with his neighbor shortly after they were born. He uses this to rationalize his crush on his sister. They may or may not have done anything sexual at that age.
In the Hallmark movie What The Deaf Man Heard, the main character's love interest mentions to him in passing that her Jerkass brother is the one who revealed to her that she was adopted. Although this is never brought up again, the reason for its inclusion in the script becomes apparent when it comes to light that the main character is the illegitimate son of her adoptive father, and thus the heir to the family fortune.
The Wrong Box: Parodied in the Dudley Moore film: when the protagonist laments that cousins cannot marry, his love interest replies that she was adopted. After a moment's reflection, he responds: "Really? I was adopted too."

Fairy Tales

In Madame d'Aulnoy's "Princess Belle-Etoile", the title character is found with her two brothers and her cousin as a baby and they are all raised by the same couple. Princess Belle-Etoile and her cousin fall in love and find it very strange that she doesn't react to him the way she does to her brothers. Then, they learn they are foundlings and set out, and so find that they are only first cousins, and marry.

Literature

V. C. Andrews
- In the real books, usage of this trope is minimal. Adopted siblings Bart and Cindy have UST, but this is never resolved.
- In the books by the infamous ghostwriter Andrew Neiderman, it's actually more common for the heroine to end up with a man she initially believes is her brother/half-brother/cousin until it's revealed that they are not actually blood-related, making it fine for them to marry each other. The best example of this is the Cutler Series where Dawn learns that she's actually adopted and that Jimmy is hence not really her brother; Jimmy even expresses his relief about this as he was attracted to Dawn even when he thought they were related. (And just to top this all off, Dawn's original boyfriend Philip turns out to be her actual brother who remains obsessed with her after the truth of their relations comes out to absolutely creepy levels.) Other examples are Heaven's daughter Annie and apparent half-brother Luke from The Casteel Series, Melody and apparent cousin Cory from the Logan Series (okay, it turns out they're still blood-related but only as second cousins, which makes marriage between them legal), and Rain and apparent brother Roy from the Hudson Series (although they don't end up together, and there's that thing about Rain's own blood brother being attracted to her too — she doesn't end up with him either, thank goodness).
Antigone and Haemon. Of course, Antigone's mother is also her grandmother, so being into Haemon seems almost normal by comparison.
Ascendance of a Bookworm: After the protagonist gets Adopted into Royalty (technically nobility), there are a few scenes in which she suspects that her adoptive father is planning to have her marry her adoptive brother. It later turns out that all the available men in her adoptive family, barring a great-uncle who is officially her grandfather by blood, are technically options for her. This includes her adoptive father, because he's stubbornly monogamous in a society that has polygynous Exotic Extended Marriage.
In The Bible, when King David's son Amnon intends to rape his half-sister Tamar, she begs him to first try marrying her legally. Some interpret that to mean the trope was in effect.
Big Trouble: Subverted (at least in the book, the ending of the movie is more ambiguous). Matt and Jenny fall in love throughout the events of the book, but the epilogue states that after Matt's dad marries Jenny's mom, dating feels weirder for them, and they eventually decide they're Better as Friends.
Brothers Conflict is a Themed Harem involving thirteen stepbrothers (the oldest being 31 and the youngest being 10). As if that wasn't enough, it's revealed that Ema was actually adopted and her father is not her biological one in order for the creators to continue shouting "IT DOESN'T COUNT!" as loud as they can.
A Villainous Incest couple who actually produced a child in A Brother's Price were considered siblings by the culture — marriages consist of one man, maybe two if they're lucky, and a band of sisters who are themselves descended from sisters, and all offspring from this marriage are considered to be siblings — but genetically are cousins. The social consideration overrode this trope and the couple was regarded with disgust and horror regardless of genes. The trope itself was used to point out that their child wasn't as poorly off as she could have been.
Brother–Sister Incest is invoked in Richard Ellis Preston Jr.'s Chronicles of the Pneumatic Zeppelin; although he was seven when adopted, and Max an equivalent age (for a Half-Human Hybrid), and Sabrina was thirteen, Romulus feels guilty about being attracted to either, because they are adoptive siblings.
In the young adult novel Cluing In by Jo Ramsey, main character Jamey Mandel's parents, Tamara and Sam, are stepsiblings; Tamara's father Victor is married to Sam's mother Martha. Somewhat subverted in that Tamara got pregnant with Jamey (at age 17) before she and Sam realized their parents even knew each other, and before Martha got pregnant by Victor. Victor and Martha got married before Tamara and Sam did, though.
In Cyteen, citizen Justin and azi Grant were raised together from the age of three. The fact that Justin still calls Grant his brother years after they’ve started sleeping together doesn’t stop theirs from being one of the healthier relationships in the book.
The Dark Elf Trilogy: Wulfgar and Catti-brie, both humans and both adopted children of the dwarf king Bruenor Battlehammer, are not otherwise related. It doesn't last anyway.
P. D. James's crime novel Death In Holy Orders featured a half-brother and sister who discovered each other's existence as adults and start a casual sexual relationship. The sister, in particular, behaves as if it's no big deal, and makes the excuse that it's not like they're full siblings. Of course.
Doctor Zhivago: Zhivago and Tonya, as mentioned above, also hit it off in the book. Although Zhivago's eyes are set on someone else…
Jim Butcher's The Dresden Files: Harry Dresden and his mysterious ex-girlfriend Elaine were adopted by their Evil Mentor when they were small children and raised as brother and sister. Once puberty struck, they took their relationship in the expected direction — which, it is also repeatedly implied, was what their Evil Mentor intended as part of his More than Mind Control manipulation of them to be loyal to him. Granted, "small children" here is about ten, at least in Harry's case, which is more than old enough to avoid feeling like brother and sister.
Inverted in Orson Scott Card's Ender's Game: Ender's speaking makes it public knowledge that Miro and Ouanda are, in fact, blood half-siblings. Given that the alternative was that Miro's dad's debilitating disease was profoundly different from every other case in recorded history in that it didn't start by obliterating his reproductive capabilities, you'd think somebody would have worked it out sooner, but the entire colony is profoundly Catholic and so the idea of his wife stepping out on him to have five or so kids just never crossed anyone's mind. It's pretty sad. Almost no one else knew about the disease until the man's death. His wife hid the records.
Odd subversion in The Firebringer Trilogy: When hero Jan's father Korr flies into a rage over Jan's marriage to the healer's daughter Tek, Jan doesn't understand, until he finds out that Tek's father was not the healer, but actually Korr. This in turn sends Jan into something of a Heroic BSOD, and when he sees Tek again and tells her what he has learned, they agree to renounce their marriage, believing themselves to be half-siblings. Just before they actually do so, Jan's mother steps in and reveals that the pair aren't actually related by blood—Korr is not Jan's father after all.
Frankenstein (1818). The same was true, but considerably more (sexually) explicit, in The Film of the Book, Mary Shelley's Frankenstein. At least kind of; Elizabeth was raised by Victor's parents, but there seems to have always been a kind of barrier between the two thinking of each other as siblings, probably because their family always wanted them to get together as adults. Note that they may be biological cousins depending on which edition you read.
In the Harper Connelly series written by Charlaine Harris, Harper develops a romantic and sexual relationship with her stepbrother Tolliver that eventually leads to them becoming engaged, much to the shock of their relatives. Making it especially squicky, they have two mutual half-siblings from his father's marriage to her mother.
In the Hetty Feather series by Jacqueline Wilson, Hetty's most consistent love interest is her former foster brother Jem. In the final book, Emerald Star, he proposes to her and when she protests that she is his sister, he tries to convince her that they are allowed to love each other because they are not blood relations. Hetty rejects him anyway as she is leaving to join the circus.
Jack Harkaway, a popular Penny Dreadful hero, fell in love and eventually married his adopted sister. They had a kid together.
In Elizabeth Peters' book The Love Talker, the main character Laurie is aware of un-sisterly feelings towards the half-brother with whom she did not grow up and in fact barely knows. She eventually learns that her supposed brother is actually an adopted child. Based on some of his earlier actions, she concludes that he knows this and has romantic intentions toward her too, which is terrific, saying something about how it's good to know because she'd draw the line at Brother–Sister Incest.
- Another Elizabeth Peters' example comes from her Amelia Peabody series: Amelia's son Walter "Ramses" Emerson is attracted to his adopted sister Nefret pretty much from the moment he meets her, but it takes Nefret a long time to see Ramses as anything other than an Annoying Younger Sibling.
In Medusa's Web, Scott's romantic interest is his first cousin Ariel, who he grew up in the same house as. It's established very early in the book that their fathers were related by adoption only and they're not blood relatives.
In Midnight’s Children, Saleem tries to invoke this when confessing his love to his sister. (He was Switched at Birth, you see.) She doesn't go for it, and in fact punishes him by abandoning him to the army after they become orphans.
There's a Mills and Boon medical romance called something like A Wife for the Baby Doctor where the eventual wife of the title is the hero's adopted sister.
The Missing: Invoked by Chip after he learns that Jonah is adopted and Jonah's sister Katherine isn't. Chip asks Jonah if he (Jonah) and Katherine can date. Jonah considers Katherine to be his sister and is grossed out by the idea.
The Mortal Instruments:
- Clary and Jace spend two whole books thinking that they're long-lost siblings, leading to much angst, until the third book. While they are not biological siblings, Jace was actually raised by Clary's father, Valentine, until he was 9 years old, making him his father by all accounts. However, Clary was raised by a single mother and doesn't meet Valentine until she is 16, notably after she meets Jace. Reportedly, the writer was going to have them be blood siblings until she decided against it.
- Midway through City of Bones, it is revealed that Alec has a crush on his adopted brother, Jace, whom he knew since they were 11- and 9-years-old, respectively. However, this is explained to be a result of his inferiority complex and Gayngst, since Jace is the closest male acquaintance he has. Furthermore, this trope is far from the only problem any romantic relationship would have; Incompatible Orientation is shown to be more of a factor against (as Jace is, as far as we know, straight) and later books reveal that parabatai are forbidden from loving each other romantically, so even if Jace does reciprocate the feelings, it would open another can of worms. After Alec experiences Character Development and becomes more open, he moves away from his attraction at Jace and begins dating Magnus instead.
In My Next Life as a Villainess: All Routes Lead to Doom!, the Claes family adopts a distant relative, Keith, to become their new heir when it becomes clear that Katarina won't be able to carry on the family name as she is betrothed to the third prince of the country. Within a few days he falls in love with his new big sister and does his best to protect her, but she's kinda clueless and never gets a hint. Keith's new adoptive mother ends up being won over by him and tries to get Katarina's engagement annulled so that Keith can marry her instead, but has no luck. What the Duke did is practically the standard Japanese practice in his position, because (as discussed in the Real Life section) in ancient Japan, titles of nobilty are strictly transmitted to sons, and not even first cousins.
Shaun and Georgia in the Newsflesh series. They're adoptive siblings and did consult a geneticist to be sure they weren't blood relatives.
Things get very complicated in Ursula K Leguin's "O" short stories: the people of O have four-person marriages called sedoretu, but because humanity is divided into two categories (moieties) which you inherit from your mother, each man is only allowed to sleep with one of the women (and vice versa)—your partner must be from the other moiety. If you're a child of one partnership, the children of the other are your "germanes", and when you grow up, you're allowed to marry and sleep with your germane (who, after all, isn't even a half-sibling, but will have grown up living in the same house). A full brother and sister can also marry, but they won't be in the same sexual pairing as they'll have inherited the same moiety from their mother. Whether or not it's considered appropriate to marry your germane or sibling seems to vary depending on where you live. It Makes Sense in Context—if you concentrate. Le Guin was the daughter of an anthropologist and it shows.
In Sharon Maas's Of Marriageable Age, Saroj finds out Nat is her half-brother, only after she's carrying his child. Of course it turns out she's adopted, in the end..
Both played straight and subverted in One Hundred Years of Solitude.
- Rebeca marries José Arcadio. Rebeca is adopted, and they didn't grow up together: Rebeca came to their family at age 11, and José Arcadio was out of the house by then, having run away with the gypsies. Yet it's kind of a Subverted Trope in that José Arcadio does seem to consider it incest. He calls Rebeca "hermanita"^note"little sister" in bed. When he goes to dump to Rebeca's Disposable Fiancé for her, José Arcadio does not pull out this trope as a defense, and instead very much owns it as incest.
  Pietro Crespi: She's your sister.
  José Arcadio: I don't care.
  Pietro Crespi: It's against nature, and besides, it's against the law.
  José Arcadio: Fuck nature two times over.
- The second example is a spoiler for the end: Amaranta Ursula (seventh generation) and Aureliano (the second) Babilonia get married under the pretense that they aren't related, but then Amaranta Ursula gives birth to a child with a pig's tail, he is eaten by ants, and Aureliano Babilonia realizes that Amaranta Ursula is his aunt. It Makes Just As Much Sense In Context
In Agatha Christie's Ordeal by Innocence, the Argyle children are all adopted and therefore have no blood relation despite being raised as siblings. Throughout the events of the books, it's made quite clear that Micky and Tina have romantic feelings for each other, and it is implied that they would eventually marry.
In the Lord John Gray spinoffs of Diana Gabaldon's Outlander series, the main character finds out that his new stepbrother was a guy he used to sleep with. It's awkward, but that apparently doesn't last long, and they're promptly back together. Which is okay, except for the fact that they're living in the 18th century and hence have the appropriate awkwardness. (There was Character Development in there somewhere, but it still managed to squick some readers...)
Last Call: the protagonist marries his foster-sister (at that point the full-fledged earthly representation of a syncretic moon goddess). Mythologically speaking, they can't really win the game (as it were) without marrying; the Squick factor is potentially mitigated (or greatly enhanced) by the fact that the protagonist was fifteen or sixteen when his sister was born and at the time of the novel hasn't seen her since she was a child.
Murder in the Mews: Double-subverted in "Dead Man's Mirror." Gervase wants his nephew Hugo to marry his adopted daughter Ruth and give their kids his surname in order to continue the bloodline. Then, it turns out that Ruth is the illegitimate daughter of Gervase's brother (Hugo is their sister's only child), so she and Hugo are biologically related, although they don't know it. Then it turns out that Hugo and Ruth are both already engaged or married to other people, but have been hiding this to avoid Gervase's displeasure.
Sad Cypres: Roddy and Elinor are raised as cousins and have been sweethearts for a long time, but are only related due to the marriage of Roddy's uncle to Elinor's aunt. Roddy later dumps Elinor to pursue Mary Gerard, but still has some feelings for her.
- The Stress of Her Regard: after the death of his wife, the protagonist is Mistaken for Murderer by her Angsty Surviving Twin. She later comes to accept that he is innocent of the crime, and for years they work together (and pass for) brother and sister. When at a later point they have to pass as husband and wife, there are complications.
In Rafael Sabatini's Scaramouche, Andre-Louis and Aline are raised together, and Andre-Louis refers to her as a "sort of sister". They are completely in love.
Played with in Augusten Burroughs' Sellevision where two main characters fall in love and bang, then find out they are related but they're actually not... whew.
A Song of Ice and Fire has this a lot, mostly due to the Westerosi custom of fostering their children. Romances between foster-siblings aren't considered anything out of the ordinary and are often encouraged, as it makes the Arranged Marriage that comes after a lot easier to go through with. Kids are usually neonates by the time they're fostered out, and view their foster-siblings as peers and friends, not siblings:
- Lord Petyr Baelish played "kissing games" with both his foster-sisters, Catelyn and Lysa Tully. He eventually fought an (unsuccessful) duel for Catelyn's hand—and later married Lysa, though Cat was the one he truly loved.
- Tyrion Lannister invokes this deliberately by sending his 9-year-old niece Princess Myrcella Baratheon to live with her fiancé, 11-year-old Prince Trystane Martell, years before they will be old enough to marry.
- Prince Quentyn Martell mentions that there was a general expectation that he would marry his foster-sister, Gwynth Yronwood. He doesn't, but it would probably have been better for him if he had…
- Theon Greyjoy—although they never interact in-series, he remarks once that he'd hoped Lord Ned Stark might marry him to his foster sister Sansa (not out of romantic love, but to confirm his connection to the Starks), even though he knew it would never happen. Later, when he takes Winterfell, he thinks that, if she weren't in King's Landing, he might have forced her to marry him to secure his hold on the castle. Made somewhat squicky since, the last time he saw her, he was 19 and she was 11, and they're only two years older now.
- In his youth, King Robert Baratheon was very enthusiastic about being engaged to Lyanna Stark, the bio-sister of his foster brother Ned, at least in part because it would have made him and Ned family by marriage.
The Stormlight Archive: Downplayed. Dalinar and Navani are legally siblings because Navani is the widow of Dalinar's brother, which makes it difficult to find someone to marry them.
Thalia's Musings: Zeus and Hera. The Titans are beings of pure spirit who paired off and joined their life force to create the first generation of Olympian gods. Zeus and Hera, as well as Hestia, Demeter, Hades, and Poseidon, were created by Cronus and Rhea.
The Cullens in The Twilight Saga — though all the "children" met as adults, and weren't raised together.
Under the Pendulum Sun: Cathy learns she is a Changeling, a replacement for the real Catherine Helstone, and thus not the blood sister of her beloved brother Laon with whom she has UST. They soon begin a pseudo-incestuous relationship. Subverted Trope when it turns out that was untrue—she's not actually a changeling, and they actually are blood siblings.
Zig-zagged in Kelly Link's short story "Valley of the Girls". The main characters are all rich teenagers whose parents have hired them Faces, essentially Body Doubles who act as their perfectly-behaved public relations stand-ins so that they don't have any embarrassing scandals (the kids themselves have implants which prevent them showing up on video or photographs). The unnamed narrator's Face is in love with his sister Hero's Face and the narrator recorded them having sex. The two only pretend that they're related for public events (although it's been their full-time job for at least eight years, which means they've spent nearly their lives faking siblinghood). But still, in all ways except legally, a Face is the person they stand in for — the narrator bitterly notes that his Face makes a better him than he does — so if anyone ever found out, it would be treated as real incest. In fact, when his sister Hero sees the video, she reacts with exactly as much horror as if it actually was a video of her having sex with her own brother. Which is why she decides to kill the narrator.
Warrior Cats: Lilyheart and Snowbush are foster siblings who ended up becoming mates. Biologically, they're cousins.
Jackie and Jaymee in We Can't Rewind end up getting married. There's no cop-out here, as the story actually begins with the groom's widower father telling how he met and married the bride's tragic single mother, making them step-siblings.
In the White Indian series, the relationship and later marriage of the second El-i-chi and Ah-wa-o is considered problematic by the elder matrons of the Seneca tribe due to the marriage of El-i-chi's mother Toshabe and Ah-wa-o's father Ha-ace.
Heathcliff and Catherine Earnshaw in Wuthering Heights (1847), Heathcliff being a fostered foundling. On the other hand, it's been speculated by fans that (like many "wards" of the era) he might really have been an illegitimate son of Catherine's father, which would make them half-siblings.
Deconstructed in Worm with Panacea, who is in love with her adopted sister Glory Girl. This is never portrayed as being any less disturbing simply because they are not biologically related, and it ends badly. Very, very badly.

Live-Action TV

A Small Light: Miep's adoptive parents bring this point up when they suggest her marrying Cas. Neither Miep or Cas are supportive of this idea, especially since Cas is a closeted homosexual.
Played With on 3rd Rock from the Sun when the Solomons attend a family reunion pretending to be long-lost relatives. Tommy falls for one of the other guests, but she freaks out because he's supposedly her cousin. Harry, meanwhile, has been catching up on family gossip and reveals that she was adopted, which Tommy expects will fix things. Instead, the girl (understandably) is too freaked out about this revelation to care about his romantic interest.
- All three male Solomons also occasionally show sexual interest in Sally — again, because her being their sister/aunt is only a cover story. Sally always shoots them down.
A very strange example in 16 and Pregnant (and spin-off Teen Mom): couple Catelynn and Tyler are stepsiblings because Catelynn's mom April and Tyler's father Butch are married. Butch and April met and married after their children started dating.
Subverted for all it's worth on Arrested Development. Cousins George Michael and Maeby are attracted to each other and George Michael clings to any hope that Maeby isn't his Aunt Lindsay's daughter. When they find out that there's a chance she's been adopted, they go to second base... then they find out that she is Lindsay's daughter. As a result, Maeby begins dating a boy from school—Steve Holt. Meanwhile, Steve decides to find the father he never knew, who turns out to be GOB, Maeby's uncle. As a result, they break up. Then, Maeby's mother Lindsay turns out to be adopted. Lindsay promptly starts hitting on George Michael's father, who is completely freaked out after thinking she was his twin for 37 years. And then, "in a kneejerk act of revenge", Michael's brother GOB decides to hit on his adoptive sister Lindsay. Subverted again in the Grand Finale with the reveal that Lindsey actually is Lucille's much younger half-sister, meaning Lindsay is GOB and Michael's half-aunt, while George Michael and Steve Holt are Maeby's half-first cousins once removed (or about as related as second cousins).
- Lampshaded with the in-series movie Les Cousins Dangereux, a stereotypical french movie about cousins in love. When it's finally re-made for American audiences, the narrator explains that "the studio [...] had hoped to avoid controversy by artlessly explaining that the two leads were not biological cousins" but it didn't stop the protesters from picketing the premiere.
- A callback joke has narrator Ron Howard remind the audience that George Michael and Maeby might not be actual cousins.
On As the World Turns, its signature supercouple, Holden and Lily, are actually uncle and niece, because her mother was adopted by his family. The ickiness is undercut by the fact that she did not grow up knowing him as her uncle—she and her twin sister, the product of their mother's rape, were themselves given up for adoption.
Battlestar Galactica (2003):
- Kara Thrace and Lee Adama are not biologically related, but they consider the same person to be their father (Bill Adama) and Kara was engaged to Lee's dead brother Zak. Kara's father Dreilide disappeared from her life when she was a little girl. Kara's and Lee's interactions wildly vary between very close, occasionally juvenile best friends and a highly-charged sexual relationship. In fact, they nearly cheated on Zak while drunk the first time they met.
- Also any sexual relations between the seven models of humanoid Cylons, such as Boomer/Cavil and Caprica/D'Anna, as well as the failed attempts at Cylon breeding offscreen (their religion mandates "be fruitful", but their tinkered biology is such that breeding with each other simply doesn't work), since they all refer to each other as "brother" and "sister" and have the same "parents" in the Final Five, but don't actually share DNA except for the copies of the same model, who are of course genetically identical. Also, Cavil is basically a clone of Ellen's father, so unless there was some sort of unspecified genetic tampering going on, they were indeed blood related.
The Soap Opera The Bold and the Beautiful:
- A particularly squicky example: In 2003, Ridge discovered that his father was not Eric, but Massimo. Meaning his half-sister, Bridget, wasn't his sister. He told Bridget and they were kissing shortly thereafter. That's bad enough on its own but consider their history. Ridge had been in an on-off relationship with her mother for 15 years at that point. He also helped Brooke deliver Bridget in Big Bear ten years ago. He and Brooke had an affair, so they all believed Ridge was Bridget's father until she was 4. She was even named Bridget after Ridge. So Ridge made out with a woman who he not only thought was his sister but also his daughter. Thankfully, any Ridge/Bridget pairing didn't get too far, but several years later, Ridge's younger "brother" Rick used the fact that they were not biological brothers as justification for pursuing both of Ridge's daughters, claiming that they were fair game since they weren't biologically his nieces. Negative audience reaction mercifully ended these stories very quickly as well.
- And a not-quite-as-disgusting-but-still-weird one, when Macy's younger brother CJ (they shared the same mother), fell in love with Macy's younger sister Kimberley (they shared the same father) and actually had to keep reminding her that despite sharing a sister, they themselves were not related.
Subverted in the 2009 TVB drama Born Rich: The family's "long-lost" half-brother is actually dead and the "brother" that they have been interacting with is actually a twisted con man who stole the dead man's identity and wormed his way in. He develops a romantic obsession with his "sister" to the point that he rapes her. Needless to say, when she discovers the truth about her "brother"'s identity and his actions, her previously conflicted feelings turn into hatred and revenge.
Despite the number of possible pairings and the age of the oldest kids when their (step)parents first met, not just subverted but entirely avoided in The Brady Bunch. It is, however, constantly mentioned in any parody of the show. It's also a subplot in the second Brady Bunch movie when they think their parents didn't actually get married.
- In a weird meta twist, nearly every actor on the show hooked up with one or more members of their TV family at one point or another.
Justin and Rebecca on Brothers & Sisters. When they met they believed they were half-siblings, but when she did a paternity test it turned out that they weren't, and then they became a couple. Which is a case of the familial relationship being changed in order to facilitate the romantic relationship. Unlike that one, however, the original plan was not to have them in a relationship, but the chemistry between the actors was so strong that it became impossible to not see them as a couple.
On Chicago P.D., Erin was raised by Voight and his wife as a foster child from age 15, alongside their biological son Justin, who was around 7. When they are adults, Justin clearly expresses a romantic/sexual interest in Erin, which she immediately shuts down.
Not avoided in an episode of CSI: Miami, where the father of a murdered woman's baby is discovered to be her brother: turns out they met and became step-siblings when their parents married (and the two kids were 18 at the time) and one thing led to another.
On Dark Angel, Max and the rest of her X5 unit consider each other siblings because they were "raised" together, although they don't share genetics. Their only biological siblings are their clones, who were in different units. (Jessica Alba's real-life brother Joshua does play Max's brother Krit, but it's unclear this is meant to imply their characters share DNA.) Zack falls in love with Max, and it Squicks her out. She wasn't too sanguine about the idea of being breeding partners with Alec, the twin of her brother Ben, either.
Soap operas do this sometimes, particularly Days of Our Lives, where almost everybody is related to everyone else somehow (puzzling out the Brady/Horton/Kiriakis/DiMera family tree is so complicated that NBC even runs a commercial spot with star Ali Sweeney claiming she can't figure it out). Lately, Max Brady (adopted son of Caroline and Shawn, brother of Roman, Bo, Kayla, and Frankie) has dated two of his female nieces, Stephanie (daughter of Kayla) and Chelsea (daughter of Bo), and nobody has found this the slightest bit strange or squicky. On such fine technicalities are the constantly-churning relationships of soap opera characters kept acceptable to the Moral Guardians.
Dexter: Although Dexter and Deb are not in the least bit involved, there was an instance where when Deb got angry with Masuka for not inviting her to Dexter's bachelor party. He said that it was men only unless she had wanted to be talent for the evening. Deb says that is gross as they're siblings, to which Masuka replies "not by blood". Dex just puts on his killing face in response. Deb also dated Dexter's biological brother, the Ice Truck Killer in the first season, although she was unaware of the connection at the time. Her own psychologist tries this excuse while suggesting that Debra's love for Dexter may be more than just sisterly, using this trope as a justification. She later fell in love with Dexter totally, which became a key plot point late season six onwards.
Eastenders: Dennis and Sharon.
Iris and Barry from The Flash (2014) have been raised together from a relatively young age and Barry thinks of Iris's dad Joe as a second father. Despite being his foster sister, Barry still has had a crush on Iris for a while. Perhaps in an attempt to lessen the potential squick factor, it's explicitly stated by Joe that Barry had a crush on her before he came to live with them and watched Barry be in love with her before he knew what love was. Nobody on the show seems to mind their attraction to each other, either.
The Fosters: Callie and Brendan are foster siblings and they follow this trope to a T; they even date secretly at one point. Because they're the Fan-Preferred Couple, the show-writers do everything within their power to prevent Callie's adoption papers from being finalized and making their sexual tension officially incestuous.
Gilmore Girls: Some fans were put off by the Like Parent, Like Spouse Love Triangle set up in the 2016 reboot, with Jess and Rory implied to still have feelings for each other, as Luke and Lorelai finally tie the knot at the end of it. With Luke as Jess's uncle and Rory as Lorelai's daughter, they are now step-cousins. The show's avoidance of the usual Unfortunate Implications, however, make this a non-issue for most of the fandom. Namely that Jess and Rory met as teenagers, they dated and broke up years before Luke and Lorelai got together, and they're both in their 30's in the reboot. And though Luke has been a part of Rory's life since she was a child, he got together with Lorelai after she left for college and while she loves him, she doesn't view him as her father.
The adaptation of Hercule Poirot's Christmas in Poirot has an avuncular variation. Harry Lee meets an attractive girl only to find out she's his niece Pilar. By the end, though, the girl confesses she's not the real Pilar (who died in a car crash) but her adventuress friend. She and Harry get together by the end. He had a thing for her in the original novel as well, but there she ended up with another man who in the series episode was Adapted Out.
Harry (both in the book and the film): So, I'm not your uncle anymore!
A downplayed version appears in How I Met Your Mother. In "The Locket", Robin and Barney are horrified by the revelation that they share a crazy cousin two days before their wedding, potentially making them Kissing Cousins. However, eventually they are relieved to learn that neither of them are actually related to the cousin by blood.
Any episode of Jerry Springer that claims that parents and children, siblings, or cousins are in love likely falls into this trope.
The Venturis and the McDonalds in Life with Derek, with Derek Venturi and Casey Mcdonald being the main characters who have a full bucket of Belligerent Sexual Tension.
Stepsiblings Boone and Shannon in the first season of Lost. Becomes downplayed in the second season with the revelation that Boone's mother never regarded Shannon as family. Or human, apparently.
Arthur and Morgana in Merlin. Morgana was raised as the ward of Arthur's father King Uther after her father Gorlois, his best friend, died. No expressed romantic interest on either side, but definite sparks and Ship Tease, and Morgana's handmaiden Gwen states that Morgana will surely be queen one day. Then in "The Crystal Cave", it's revealed that they are blood siblings, as Uther had an affair with Gorlois's wife! Though technically, that's been an It Was His Sled for several hundred years now.
On one episode of the new Mystery Science Theater 3000, Ator declares he wants to marry his sister Sunya. His father says it's OK because he was adopted, and nobody in the movie notices the squickiness of the fact he thought she was his sister. It's not lost on the crew, however, and they spend the episode making jokes about it.
In Ninja Turtles: The Next Mutation, it was claimed that none of the Turtles were actually related to each other; they just happened to be the same relative age in the same terrarium at the time. This was done so that there would be the potential for Leonardo, Donatello, Raphael, or Michelangelo to have a relationship with the female fifth turtle Venus de Milo, who was established as having mutated with the other four before being separated from them.
Outlander: Marsali and Fergus are Jamie Fraser's stepdaughter and adopted son who both consider Jamie their father figure. However, they lived with Jamie nearly a decade apart and weren't raised in the same household or as siblings, so they don't see each other as brother and sister. But given that this is a time period where first cousins still marry with no controversy, stepsiblings with no blood relation probably wouldn't raise any eyebrows.
Perhaps the sickest and overly complicated example imaginable on Passions. Chad who is biracial comes to town and eventually dates Whitney, the daughter of Eve and TC (the only black family in town). Chad is for several years hinted to be the son of Eve and her white lover Julian from before she was married. Later Chad and Whitney have a baby together and the revelation comes out that they are half-brother and half-sister and their baby was born from incest. Later it's revealed that Chad is not really Eve and Julian's long-lost son. It turns out Julian's father Alistair had raped Eve's adopted sister, so Chad's son is not a product of incest - but on the other hand, Chad himself is a product of rape. Even later it is revealed that Chad is having a gay relationship on the down-low with a guy named Vincent... who turns out to be Eve and Julian's hermaphroditic long-lost child for realsies this time... oh and he's a serial killer and a rapist to boot. So yeah Chad didn't sleep with his half-sister... but he did sleep with his girlfriend's half-brother who was also his own half-nephew.
Not canon, but this is the rationale people who 'ship Brooke and Sam of Popular together use. That and the fact that they were about 16 when their parents married.
Pretty Little Liars: Stepsiblings Jenna Marshall and Toby Cavanaugh. The main characters quickly assume it wasn't consensual and that Toby was the perpetrator, since All Abusers Are Male. The Plot Twist is that it was coercive, but Jenna was the perpetrator.
Princess Silver: Played with. Rong Le wakes up with amnesia and is told Rong Qi is her brother. She continues to think he's her brother for most of the series until she regains her memories. Then she learns they aren't related at all and were lovers before she lost her memories. Rong Qi's mother came up with the idea of making Rong Le think they're siblings to keep them apart.
In the Season 2 premiere of Reaper, Sock, once he gets over the shock of a stranger living in his mother's house, instantly lusts after his hot Japanese stepsister whom he just met when they were both adults, and did not grow up together. The problem is, she just wants a genuine sibling relationship between them and is completely oblivious to his attraction (until it gets really, really Anvilicious).
Roseanne:
- Darlene and David's relationship didn't start this way, but it became more and more the case as the series went on. When David was "adopted" (taken in) by the Conners, Roseanne let him know he could move in by walking in on David and Darlene kissing and cheerfully crying to Dan, "Oh, look, honey, our kids are necking!" By the time they got married, Jackie opened the episode by saying, "I can't believe our little Darlene is marrying... our little David," Mark scolded David for asking about their birth parents when the Conners were practically his parents, and David and Darlene paid tribute to "our parents Dan and Roseanne Conner" in their wedding vows.
- Another episode centers on David being creeped out that he had an erotic dream about Roseanne because he thinks of her as his mother.
Adrian and her stepbrother Max in The Secret Life of the American Teenager.

Two examples in Shadowhunters, both involving Jace Wayland:
- Jace and Clary Fray begin a relationship, then discover that they are actually brother and sister because Big Bad Valentine Morganstern is the father of both of them and immediately end the relationship. Later on, it turns out that they are not biological siblings at all, as Valentine, who is Clary's biological father, kidnapped Jace as a baby. So Jace was raised by Michael Wayland who was really Valentine but isn't related to him by blood, and Clary is related to Valentine by blood but wasn't raised by him.
- Alec Lightwood initially has a crush on Jace, who has lived with Alec's family since Jace's father died (or rather Valentine faked his own death) when he was 10. Jace does not feel the same way, both because he is straight and because he considers Alec his brother. Alec eventually comes to regard him as just a brother as well, and enters a committed relationship with Magnus Bane.
This trope comes up in Silent Witness in the storyline "Domestic". It's revealed that shortly before she and most of her family members were killed, a woman discovered her teenage daughter and stepson were sleeping together. She was extremely distressed by this and informed her husband, insisting they put a stop to it. However, her husband didn't see what the issue was because their children aren't actually blood related, and he and his second wife married and blended their families a few years ago, as opposed to their kids being raised together as siblings. The woman then revealed the Awful Truth; she's known for years that her stepson is not actually her second husband's child but was in fact fathered by her first husband, who had an affair with the second husband's now-deceased first wife. This means that the supposed stepsiblings are in fact half-siblings. The husband didn't take this well.
Nate from Six Feet Under has an affair with his newly introduced stepsister. Nate is neither squicked by this fact nor the fact that he's screwing around on his wife with someone he's known for less than a month. This, plus Nate's trope-naming Karmic Smackdown had Nate's portrayer, Peter Krause, not happy at all.
Smallville featured an example on the alternate-universe Earth-Two: Tess Mercer is having sex with her adopted brother, Clark Luthor. When you think about it, the boy was adopted, and the girl was never taken care of by her father. So even if both have Lex as a brother in one way, they are hardly siblings in any way.
Likewise completely avoided in the ABC sitcom Step by Step. Though considering how... unBrady-like their relationships were amongst themselves, this is somewhat more understandable.
In Terminator: The Sarah Connor Chronicles, as far as anyone outside the Connor family knows, Cameron is John's sister. The sexual tension, by the way, is palpable.
Subverted in That '70s Show, in an episode where Eric's hot cousin comes to visit and tells him she's adopted, and then seduces him just to trap him into making a move on her in front of his parents. She's not adopted at all; she just wanted to pay him back for a prank he pulled when they were younger.
On Two and a Half Men, Charlie becomes sexually involved with the daughter of his mother's fiancé, repeatedly pointing out that she would only be his stepsister.
Ultraman:
- Most of the Ultra Brothers are this. However, it is explained that they are all distantly related to each other in some way, usually as part of extended families. Zoffy, for example, is the King's grandson, and Ultraman Ace is one of Ultraseven's cousins.
- This trope is played around with Ultraman Ace and Taro — whilst Ace is an orphan who was adopted by Taro's family, they have such a close brotherly relationship that they might as well be considered blood brothers.
- This is averted by Ultraman Leo and Astra, who are blood brothers by all accounts. This status is clearly emphasized by Astra's tendency to yell "NII-SAN!" to Leo whenever he is in danger or being beaten down.
The Umbrella Academy: First, a little background for all these examples. There was a Bizarre Baby Boom and 43 superpowered kids were born to women who were not pregnant when the day began. These kids are presumably biologically unrelated.^noteAlthough — given the weird circumstances of their births — there might be some possibility that they're paternal half-siblings?
- Reginald Hargreeves adopted 7 of them and raised them together from infancy. They're mostly considered family and are constantly referred to as brothers and sisters, yet there was also a cold, institutional, un-familial aspect to their upbringing as well. They grew up in a hermetically sealed environment and weren't allowed outside friends. Allison and Luther had a childhood crush on each other. When the show picks up, they're all 29 and have come home for Reginald's funeral after not seeing each other for years. At this time, Allison and Luther's old crush reemerges. In the first season it's given a more romantic treatment, but due to fans not liking it, the following seasons give it increasingly cynical treatment. Thematically, their relationship is emblematic of the highly insular nature of the Umbrella Academy. When Allison and Luther were kids, it was Situational Sexuality — who else was there to have a crush on? Now that they're adults and out from under Reginald's thumb, they can form relationships with outsiders if they so choose. For them to not do so, and instead look to each other for this, is an expression of arrested development.
- In Season 2, Diego falls in love with a girl who turns out to be another one of the Bizarre Baby Boom children. When he learns of this, he nervously asks the others if they're not biologically related. This particular superpowered kid was raised by different parents.
- In Season 3 they go to an alternate universe where Reginald still adopted some Bizarre Baby Boom babies, but they were different kids this time. Luther falls in love with one of them. They have the same adopted father, but they were never raised together.
In Season 1, Veronica Mars finds out that her mother and the father of her ex-boyfriend Duncan Kane were High-School Sweethearts who may have continued their relationship as an extra-marital affair... meaning that she and Duncan may or may not be half-siblings. This revelation leads her to pull her car over and puke on the side of the road. Luckily, the paternity test that her father has done proves that she and Duncan aren't related. A few months later, they resume their relationship. Though according to Word of God, Duncan was SUPPOSED to be Veronica's half-brother for real, but the network wouldn't let them put Actually-Blood-Siblings sex on the air. So it was originally meant to be a complete (and squicky) subversion of the trope.
In Wonderfalls after Mahandra and Aaron sleep together, she freaks out because they grew up together although they're not actually adopted siblings.

Music

Dennis Warner's song I'm My Own Grandpa plays this for laughs.
Lord Melody's Shame & Scandal too. Huge in France.
Played straight, but in a twisted way in Johnny Be Fair, by Buffy Ste. Marie. Video here.
In Vocaloid:
- Rin and Len Kagamine, who fans are debating over whether or not they are twins... despite the fact they have the same last name. Some claim that they are merely only genderbends of each other to avoid having their fanfictions labelled as Twincest (and thus it's selfcest instead), while others are eager to embrace the twincestiness that comes with them.
- To a lesser extent is the genderbents of the vocaloids (i.e. Mikuo, Kaiko, Gakuko, Luki, Ted); the fandom can get away with pairing them up because they are simply gender reversals of the official vocaloids and therefore are not canon siblings.
- In an attempt to placate the detractors, the creators declared that Rin and Len were literally mirror-versions of each other, so probably somewhere between Half-Identical Twins and Opposite-Sex Clone. Of course, considering that all the Vocaloids are computer programs...

Puppet Shows

Shiro and Lamia in Star Fleet. Lamia was sent to Mars from the Planet Esper to escape Imperial Alliance attack, where Professor Hagen, Shiro's father, raised her in secrecy.
Lady Katsura and Lady Hatsuhana in Tamamo-no-Mae Asahi no Tamoto. Even though Katsura is adopted, they seem to be on perfectly good terms, as Hatsuhana is willing to be sacrificed instead of her sister. Too bad Katsura is killed off anyway.

Theatre

The Guilty Mother by Pierre Beaumarchais (the less famous of the two sequels to The Barber of Seville) revolves around a pair of young lovers: Leon, the son of the Count and Countess Almaviva, and Florestine, Almaviva's ward. It's revealed that Florestine is the Count's illegitimate daughter, which would make her Leon's half-sister — but then it's established that Leon is the product of an affair and the Count is not his biological father, meaning that he and Florestine are not related after all.

Video Games

Arno and Élise in Assassin's Creed: Unity. Arno's father was assassinated when he was a child and was adopted by Élise's father. Although they grew up together, neither of them saw the other as a sibling and they started a relationship sometime in their adolescence that flourished until Élise's death.
Baldur's Gate:
- Fans frequently debate whether a relationship between the PC and Imoen, both Children of the dead god Bhaal, would "count" as incest. Those who favor the relationship usually point out that the PC is often a different race from her, and that canonically they are known to not have the same mother so that their main connection is definitely not any mortal source. (This is even brought up in the expansion Throne of Bhaal in a discussion between her and one of the other party members; Viconia suggests she should serve the main character in every way he desires for all that he's done for her, and why it shouldn't be a problem. Imoen balks at the thought and finds the idea disgusting... and then turns the question back on Viconia, who's also been canonically saved twice by the protagonist and is a romance option). Ultimately irrelevant, as she isn't a romantic option in the game... unless you mod it.
- The same argument is made for the female PC and Sarevok, who are also Children of Bhaal from different mothers. It's a little easier to defend, since Throne of Bhaal makes it clear that Sarevok no longer has the Essence of Bhaal, and that was how he was related to her. Once again, it's irrelevant since he's not a vanilla romance option.
The plot for the Kingdom of Caerleon in Brigandine involves King Cai falling for his sister, who turns out to have been adopted.
Claimed in the penultimate route of Duel Savior Destiny, but then undone in the actual final route. The story first claims that it's so convenient that Taiga and Mia aren't related in the resolution to her route, but in the Crea/harem route that follows it, the story notes that they have a blood relation. Whether this is a subversion^note or a simple retcon is unclear.
In ENIGMA: An Illusion Named Family, one of the siblings is actually adopted. Learning this led one of the other siblings to Blackmail them, threatening to reveal the adoption and that she's been sending money to her birth parents. Another sibling feels that this justifies his secret attraction to her.
The curate and his [adoptive] sister in the online game Fallen London. It is possible for you to seek romance with either (or both) of them, and during the process, you can find what looks like a fragment of a love letter from the sister to him.
Final Fantasy:
- Final Fantasy II mostly contains only hints of love interests but the No Export for You novel does have Maria harboring feelings for her adopted brother and the story's main character, Firion.
- Inverted in Final Fantasy VIII: When Quistis suddenly remembers that she was raised in an orphanage with Squall she stops hitting on him. In fact, Selphie and Irvine are the only ones who actually remain inside their raised-together group for romance. Everyone else from the orphanage looks outside the group for romance, as Zell hooks up with Library Girl and Quistis gets shafted.
Fire Emblem:
- The series tends to prefer the full-blooded version of this trope, but apparently decided to go the Not Blood Siblings route in Radiant Dawn... twice. Once with Micaiah and Sothe, and also with Elincia and Geoffrey. In both cases, they end up married at the end if they have an A-support with each other. Micaiah and Sothe is a combination of this and reversed Wife Husbandry. Due to her long life span, she raised Sothe from when he was a kid, though it could be argued that she viewed him as a little brother since physically she is only in her mid to late teens. Interestingly, they are the only characters who start out with A support for each other.
- In the original Fire Emblem Gaiden, the relationship between Alm and Celica could be seen as this, considering they spent their childhood together under the care of Mycen. Downplayed in the remake, where the game quickly notes through dialogue that they are not related.
- Near the end of the first half of Fire Emblem: Genealogy of the Holy War, King Trabant of Thracia kills Quan and Ethlyn and takes their daughter, Altenna, to raise as his own. She ends up falling in love with Trabant's biological son Areone. There's also a bit of the full-blooded version there too: Areone is a descendant of Dain, and Altenna is of Njorun... Dain and Njorun were Brother and Sister. Note that this was a little over a hundred years ago.
  - Meanwhile, Altenna's actual younger brother, Leif, is raised by Quan's loyal knight Finn. While Genealogy of the Holy War allowed a bit more freedom in pairing non-Lord units than most other early Fire Emblem games, its Interquel Thracia 776 suggests that his canon love interest is Nanna... and also that Finn is canonically her father. (Later material contradicts the latter assertion, saying that Nanna's father is whoever you chose to marry Lachesis in Genealogy of the Holy War and that Finn was more of an adoptive father in the event that he isn't her biological father, but the trope still applies.)
- In Fire Emblem: Awakening, male Robin can marry Aversa, the indentured servant of Robin's biological father, Validar. They never met each other before the events of the game and Validar never considered Aversa's family, but this doesn't stop Aversa from calling Robin "Big Brother" as a joke, which Robin detests as he they aren't related in the slightest as her relationship with Validar was purely professional.
- Doable in Fire Emblem Fates if Corrin marries any of their siblings from Nohr, since they were merely raised together and not biologically related, or their Hoshidan siblings, since it turns out they're not related either.
  - Also possible if Azura marries either of the Hoshidan brothers, as she spent most of her life with them but isn't related by blood. Meanwhile, her marrying the Nohrian brothers falls under another related trope.
In Hades it is eventually revealed that Zagreus is the son of Hades and Persephone and not Hades and Nyx like previously assumed. This puts his relationship with many of his assumed siblings, including Megaera and Thanatos who are potential Love Interests, in this light.
I Was a Teenage Exocolonist: This gets briefly discussed if Sol suggests that Tammy's surrogate mother could have been Cal's mother Tirah. Since Cal and Tammy are a Puppy Love pair who become the first people to concieve a child in the colony if Tammy doesn't die as a child, Tammy is quite quick to point out the reasons why her surrogate mother can't be Tirah. This implies that the idea of having been carried by the same woman as Cal makes her uneasy, even if they are otherwise not genetically related at all.
Yuri and Kira from Infinite Space insist that they are siblings while everyone around them thinks they're just very close friends who have a thing for each other. Yuri even gets someone to do DNA tests that reveal they have no blood relationship. They're Artificial Humans created by extra-dimensional aliens and they're only "siblings" because that's the first relationship that popped into Yuri's head at the beginning. A previous scene established that he had no relatives at all to leave behind when he went into space.
The Alex and Luna, the hero and heroine of Lunar: The Silver Star, grew up together after Luna was taken in by Alex's parents as an infant. The game makes it obvious that they are destined to be together, and no one in the entire game, even Alex's parents, is anything other than happy for them.
In the Mega Man Legends universe, Rock (Mega Man) is literally Roll's adopted brother, yet romantic hints exist. Fans debate whether the original series Rock (Mega Man) having a relationship with Roll would count as incest, as their original incarnations are both robots and no genetics are involved, despite being "siblings" created by the same man. Roll's battle with Copy Roll in Mega Man Powered Up supports this trope as Roll's copy says that she'll confess Roll's feelings to Rock. MegaMan NT Warrior Rock and Roll have no familial relation at all.
Emma Emmerich, in Metal Gear Solid 2: Sons of Liberty, has a thing for her big brother Hal "Otacon" Emmerich. Turns out that they're really just stepsiblings; they have the same last name because Hal's dad adopted Emma after he married her mother. The sexual relationship which developed between Hal and Emma's mother, however, adds an extra layer of Squick to the situation.
Matoi and the Guardian in Phantasy Star Online 2 are both revealed to be ArtificialHumans that were created by Xion, through different methods and at different points in time. Moreover, the Guardian was originally created by Luthor, but he abandoned them when he found that they were not to his standards.
The Prince and his bodyguard Lyon in Suikoden V, as she's an orphan raised by his father.
In previous versions of the game Summertime Saga, the characters the Main Character could pursue included some of his family members. Retcon changed them to roommates... less than smoothly. Besides their Strong Family Resemblance, they still treat him and each other like family members. MC describes Debby as "my father's lifelong friend" in the intro, but she was clearly in a relationship with him (she compares MC's penis favorably to his late father's), which just makes her a Lecherous Stepparent instead. His non-sister Jenny also mentions a few times that she's known MC since childhood as a reason she doesn't want to date him.
A non-Squick example for Super Robot Wars: the Kobayashi sisters Aya and Mai are revealed to be this. Both were originally test subjects, but their "father" Kenzo took them in as his daughters in the hopes their exceptionally potent Psychic Powers could complement each other for the combining Super Robot SRX.
- A similar non-Squick instance for Joshua Radcliffe and Cliana Rimskaya of Super Robot Wars Destiny: Cliana was adopted by Joshua's father and throughout the game, the two treat each other Like Brother and Sister, except Joshua is sometimes called out for his Big Brother Instinct by insisting Cliana stay away from the battlefield. Then again, when your adoptive sibling has a slightly, unstable Split Personality, you'd probably do what Joshua does.
Tales of Legendia, in which Senel and Shirley pretend to be brother and sister (arguably, they do rather act as though they are, but they certainly don't resemble the other in the slightest, and have different surnames, to boot) until they are exposed by the main villain as not being related at all—in fact, each of them belongs to a different race altogether. Not too long after this, Shirley confesses to Senel that she is in love with him. They presumably end up engaged via the Rite of Feriyen by the end of the Main Quest... although afterward, the other characters note that the relationship between them doesn't seem to have changed much at all (they even get new titles to reflect this; Senel obtains "Still A Brother" and Shirley obtains "Sister Girl").
Trails Series has two examples.
- Trails in The Sky: Joshua was brought in by Cassius Bright at the beginning of the game (around 5 years prior to the game's timeline) so that Cassius can raise him as his adoptive son and his daughter Estelle's adoptive brother. Joshua's feelings toward Estelle are subtly hinted throughout the first game and Estelle slowly realizes her own feelings too, except for a certain problem: Joshua is an assassin from Ouroboros sent to assassinate Estelle's father and then to unknowingly spy on Bright Family after the expected failure. The sheer guilt he has makes Joshua leave his families and friends including Estelle right after she confesses. Thankfully, they get better.
- Trails of Cold Steel: Elise Schwarzer is in love with her adoptive brother Rean Schwarzer, the main character of this game. The feeling is one-sided since Rean is oblivious to his sister's feelings. Unlike Estelle and Joshua, she doesn't bring it further and this trope becomes one of many sources of comedic moments especially from her best friend. She finally manages to confess her feelings in Cold Steel IV though whether Rean reciprocates or not depends upon the player.
Vandal Hearts 2: In a Bittersweet Ending (by letting your childhood friends die), you make good on the promise you made to Clive to take care of Rosaly, your adopted sister, and the ending is that she gave birth to your child. In a semi-justification, Joshua (main character) was adopted when he was almost to his teens and was separated from Rosaly for a long time over the course of the plot.

Visual Novels

Akaneiro Ni Somaru Saka hints at this with some Plot-Based Voice Cancellation, adding a considerable cop-out factor to an already frustrating ending.
Amnesia: Memories has this be the relationship between the heroine and Toma in Spade World. Toma explains to a concerned Ikki that his mother and the heroine's father got married, making the two of them stepsiblings. Eventually, the marriage fell apart and they got divorced, taking away the legal relation between Toma and the heroine. Nonetheless, the two still consider each other siblings and treat each other like that.
In Battle Construction Vehicles, main character Hayato and his co-worker Kyoko have some Belligerent Sexual Tension going on before it's revealed that she's actually the daughter of Ume, the Big Bad of the story. A little while later, it's revealed that Hayato is his son from a different mother, making them half-brother and sister. Later still, it's revealed that Kyoko was actually adopted and, sure enough, they get married in the epilogue.
Hiroki and Elis in Canvas 2. She's actually his cousin, but she's stuck very firmly in the 'little sister' category when thinking of love interests. He writes it off as a Precocious Crush or her just joking around despite her being persistent about it to Single-Target Sexuality levels.
In Crescendo (JP), this is also an important plot point. Ryo is living with his adopted sister, (their parents having died three years earlier) when his birth mother asks him to move in with her family. How the player answers the question is a major branch point in the story.
The canonical routes in both Da Capo games have always been about Not Blood Siblings, though the second game strayed into a fine line between this and Kissing Cousins for both Asakura sisters, since the protagonist is both adopted and raised by another parent—who, in turn, is another Not Blood Siblings with Asakura grandpa.
- Which makes his surrogate son Yoshiyuki the adopted distant uncle of the Asakura sisters, which make his relationship with either sister one generation apart... No, wait, that's not right. Hmm... Try to wrap your head around the family tree of this franchise.
Fate/stay night:
- Although it's pretty much one-sided, there's Shirou Emiya and Ilyasviel von Einzbern. She is the biological daughter of his adoptive father and has an obsession with him that is at least partially romantic. The fact that Word of God has openly stated that she was nearly the fourth romanceable heroine of the game and there are two separate Bad Ends where she magically makes him her slave for life (complete with heavy implications that he is 'that kind' of slave) really doesn't help.
- It is eventually revealed that Shinji Matou has been regularly raping his adopted sister Sakura.
A huge theme in the tear-jerking Kana: Little Sister.
In Obey Me!, Asmodeus's brothers aren't exempt from his flirtations (especially not Beelzebub), though none of them take him up on his numerous offers. As fallen angels-turned-demons originally created by God, they aren't actually related, although with Asmo being the Avatar of Lust whether a blood tie would stop him is up for debate.
- A variation occurs when it's revealed that the Player Character is the distant descendant of the brothers' deceased youngest sister, Lilith. With Lilith having been an angel and therefore similarly unrelated to the brothers, the MC being descended from her human reincarnation, and centuries having passed since Lilith's human life naturally ended, the brothers are nothing but overjoyed by the revelation and continue to pursue the MC as Love Interests.
Satoru, the protagonist of Riddle Joker, and his sister Nanami were both adopted (from Parental Abandonment and Abusive Parents respectively) when they were quite young. Nanami has developed a crush on Satoru as basically the first person (along with their adoptive father Ryuunosuke) to treat her kindly. How far this goes depends on the route the player chooses.
Akane and Yoshikazu in Suika. Definitely doesn't go the usual way this trope normally does. You are given the choice to supposedly go that way, but then comes The Reveal...
Kyouya in Triangle Heart 3: Sweet Songs Forever has Miyuki (actually his cousin, which permits her into the Unwanted Harem), as well as Nanoha. Nanoha's own series, which barely shows Kyouya instead of giving him a lead role, doesn't even mention this, and instead chooses to briefly tease a potential relationship between Fate and her brother (by virtue of his mother adopting her) Chrono. Although this too was abandoned in the third season when Chrono married his childhood friend, Amy. And Fate becomes the second mother to Nanoha's adopted daughter Vivio, anyway.
An integral part of the plot of some arcs in the Tsukihime game. Shiki and Akiha met when they were seven and six years old, respectively, lived together for two years, and then separated for eight, somewhat short of "growing up together." Akiha knew the whole time that Shiki wasn't her real brother, and Shiki was at least vaguely aware of it, even admitting to himself once that his protective instinct towards her didn't really seem like sibling love. Needless to say, that really only comes into play during Akiha's arc. The fact that they are not related is important in all the arcs due to the events that occurred ten years with SHIKI, the real brother of Akiha.

Web Animation

The ACTUALLY HAPPENED video I'm Dating My Stepbrother is about a teenage girl named Lindsay who's in love with her new brother. Despite the title, he's actually her adopted brother, not her stepbrother. The two end up in a Secret Relationship together.
ATTACK on MIKA: Rin starts to put the moves on her brother Daiki when she finds out they're not blood related. At the end, they get together after Daiki finds out that his online game partner, "Strawberry Girl", is actually a boy.

Webcomics

Parodied in this◊ ad for J-List by Dan Kim of Clone Manga.
El Goonish Shive:
- Approached from the opposite direction by Grace, who, despite being "practically married" to Tedd Verres, claims Tedd's father as her adoptive father.
  Mr. Verres: ...but you're my son's girlfriend.
  Grace: We're a strange family.
- Averted pointedly when Diane meets Noah. He's the adopted son of her biological father who she barely knows. She's still absolutely horrified when he points out how the way she tackled him looks, and shudders when Lucy asks if he's an ex-boyfriend of hers.
Experimental Comic Kotone: Konstantin seems to think this is going on between Onii-chan and Kotone. It isn't. What none of the characters except Kotone-chan herself and Onii-chan know is that Kotone is a Robot Girl—the whole "little sister" thing is just a cover.
Brought up by Monette in this Something*Positive comic, where she ask Davan whether it would be illegal for them to have sex as adopted siblings.
In SwordCat Princess, Erica has had sexual relations with both of her adopted brothers: Jack, who she was raised with (and who she calls "sisterf***er at the end of this scene), and Nicky, who she was not raised with.

Web Original

The heads of the GameFAQs group "The yusketeers" Yusiko and Yukito can't have a conversation without sexual tension showing up somewhere. Apparently, people have a Venn diagram just to tell them apart. Some say that the sheer sexual tension that permeates any room the two are in is enough to stun a horse.

Mickey and his stepsister Audrey on HomeSchooled.

Western Animation

This is presumably what was intended with Two-Tone and Lucky in 101 Dalmatians: The Series. They're love interests despite being siblings. In the original film they're related by blood. It's most likely the creators of the cartoon mistook Two-Tone for one of the adopted puppies.
The amount of Ho Yay between Gumball and Darwin in The Amazing World of Gumball is...well, amazing. Especially in "The Bros", when Darwin gets jealous of Gumball's actual girlfriend. They're adopted brothers (Darwin used to be the family's pet), though what arguably makes it worse is that they've lived together since Darwin was two and Gumball was four.
Once in American Dad!, Roger tricks Steve into believing that he was kidnapped from another family, causing him to kiss his supposedly-unrelated sister Hayley.
Roger: Everything that happens from this point is just gravy.
On Gargoyles, the title species lives in large clans that are considered to be a single family—nobody keeps track of who their biological parents are, and thus all gargoyles of the same age group refer to each other as their "rookery brothers/sisters" and older members as their "rookery parents." Their mates are usually rookery siblings. With there being many rookery siblings in each generation, naturally they don't all become equally close or in the same way. Word of God says that real incest never occurs, though, because a) each female can only lay one egg per mating cycle, so rookery siblings never include actual siblings except in the rare case of identical twins from the same egg and b) pheromones prevent Kissing Cousin relationships.
Jellystone!: Show artist Melody Iza has stated that Jonny and Hadji are married. Unfortunately, the crew apparently forgot that they were brothers in the original. Adoptive brothers, but brothers nonetheless.
The Legend of Korra has implications of this. Suyin says multiple times Kuvira was like a daughter to her, though she never really shows it, and Kuvira is engaged to one of Suyin's biological sons. It's never specified if Suyin adopted Kuvira or not however it's implied she was raised alongside her family from a young age.
While Adora and Catra from She-Ra and the Princesses of Power were raised from early childhood by the same Evil Mentor, Shadow Weaver, whom they both saw as a maternal figure, their relationship has always straddled the fuzzy lines between Childhood Friends, rivals, and love interests. At the end of the series, they become an Official Couple.
An odd example: in one episode of The Simpsons, Homer's father briefly dates Marge's mother. Homer is against their relationship since he believes this will make him and Marge siblings, and retroactively turn the kids into freaks "with pink skin, no overbite, and five fingers on each hand!" The two wind up breaking up, though. However, Marge later becomes Homer's aunt when Abe briefly marries Selma. This is even weirder when you take his relationship with Mrs. Bouvier into account...
Sym-Bionic Titan: Lance and Ilana, masquerading on Earth, pretend to be brother and sister at their high school. There's plenty of "moments" between them. When Lance reluctantly agrees to go to prom as a group, he decides to invite Ilana if he has to take someone (she points out she's already going with someone). One wonders how they would explain this to the others. Also, their neighbor Barb seems disappointed when she thinks that Lance and Ilana are brother and sister rather than boyfriend and girlfriend. Word of God though intentionally wanted to avoid romance between them mainly to avoid the obvious squick potential.
In Teenage Mutant Ninja Turtles (2012), Leonardo is attracted to Karai, who turns out to the kidnapped biological daughter of his adoptive father, Splinter.
On Tiny Toon Adventures, if Babs Bunny and Buster Bunny didn't always say "No relation" after introducing themselves, most viewers probably would have assumed they were siblings. This was parodied when they cameoed in an episode of Animaniacs about Noah's Ark.
Noah: And you are...?
Buster & Babs: Buster Bunny and Babs Bunny. No relation.
Noah: Let's hope not. This is a kid's show.
On Ugly Americans, Mark and Callie raise the latter's younger sister, Lillith (which admittedly only took about two weeks). Lillith then declared her intention to kill Callie and have sex with Mark in front of a stadium full of other demons. Mark, who had spent the entire episode acting like a doting, enthusiastic father, is understandably freaked out.
In WordGirl, Becky was adopted into the Botsford family. Her brother TJ has a crush on her superhero persona WordGirl. TJ doesn't know her secret, but Becky still thinks it's gross.

Real Life

Simpua or tongyangxi was a tradition of Arranged Marriage dating back to pre-modern China — particularly Taiwan — in which a family would adopt a pre-adolescent daughter as a future bride for one of their pre-adolescent sons. The children would be raised together, and then marry when they were old enough.
At present, Japanese law allows adopted siblings to marry, which has feudal-era origins. Japanese family inheritance traditionally emphasize the transmission of family traditions rather than blood, so at the time titles could only be transmitted to the title-holder biological or adopted son; even nephews needed to be adopted as an heir. This caused adoptions in feudal Japan to be as political as Arranged Marriage, and many just do both — adopting a child from another family as one's heir and having the latter marry his own daughter.

As an example: Matsudaira Katamori, ninth and the last Daimyo of Aizu and the direct boss of Shinsengumi, is an example of this and Kissing Cousins. He was born the seventh son of Matsudaira Yoshitatsu, the tenth Daimyo of Takasu, whose biological brother (i.e. Katamori's uncle) Takakata was already adopted out and became the eighth Daimyo of Aizu. When Aizu, again, was facing the problem of a lack of male biological issue, Katataka adopted Katamori, his biological nephew, as his son and by extension, the future ninth Daimyo of Aizu. To cement the relationship between the two houses, Katataka arranged Katamori to marry his biological daughter, i.e. Katamori's first cousin by blood and sister by adoption.
The Roman emperor Tiberius married his stepsister Julia as part of his stepfather's plans to make him his heir. Given that Julia's father, Augustus and Tiberius' mother, Livia had already been together when Julia was born, and Tiberius was already 14 at that time, it's still more than a bit squicky. But to their credit, it wasn't their idea (in fact, Tiberius was already married to Vipsania Agrippina, whom he loved dearly, when Augustus had this idea, and did not want to divorce her; he never got over being forced to leave her for Julia) and it didn't last long.
A variation: prior to same-sex marriage being recognized in the U.S., some same-sex couples would get around visitation restrictions in hospitals by having the older partner legally adopt the younger one, enabling "child" to care for "parent" or vice versa. This presented some complications when it came time to actually get married.
- This was apparently relatively common in Japan

Not Blood Siblings

Examples:

Alternative Title(s): Not Blood Related

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Not Blood Siblings

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