My Sister Is Off-Limits

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"And then my dad! And then my mom! And then my second-degree cousin..."

"GET OFF MY SISTER!"

— Ross finding out Monica and Chandler are sleeping together, Friends, "The One Where Everybody Finds Out"

If you are friends with a man for more than 24 hours, then dating his sister is off-limits without asking permission, and sometimes forever. Some reasons behind this are:

Whenever seeing the sister, the boyfriend can only think of the brother.
It jeopardizes the friendship — if the friend and the sister break up, the brother is put in a very awkward spot and doesn't want to betray his sister or his friend by taking a side.
The boyfriend is a shameless philanderer who treats women like his personal playthings, and the brother doesn't want his sister getting the same treatment. Alternatively, the brother is, and assumes the boyfriend is as well.
The brother is jealous of any guy who gets too close to his sister because he doesn't want to lose the position of #1 person in her life. The Incest Subtext (if any) of this one varies due to factors such as the brother's age and exactly what he thinks being "#1" entails.

Not uncommonly, the sister tells her brother to get lost, or otherwise actively ruins his attempts to prevent the potential boyfriend from dating her. After all, she may not take kindly to anyone — not even her brother — trying to control who she can and cannot date. This may lead to the brother trying to save face with If You Ever Do Anything to Hurt Her... and then backing off.

It's Truth in Television, as most any guy with a sister can tell you.

While the title says "sister," variations of it also include mothers and other female relatives (such as cousins), and for girls, the Gender Flipped "My Brother is Off-Limits." This doesn't include daughters or sons, because Boyfriend-Blocking Dad and My Beloved Smother, respectively, already cover those. Exes fall under The Bro Code instead.

This may also be a symptom of a character having a Big Brother Instinct. Could lead to a Knight Templar Big Brother. This trope is almost indispensable for any Brother–Sister Incest works, where it builds the initial Incest Subtext in story. Contrast Best Friends-in-Law, where the guy is all too happy for his good friend to end up with his sister. A My Beloved Smother might have the same feelings towards her child. Unrelated to Big Brother Is Watching. (At least, we hope so...)

Examples:

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Anime & Manga

Skuld from Ah! My Goddess flies into rage whenever Keiichi gets too cozy with Belldandy. Later in the manga she seems to grow out of it, mostly.
The Black Butler manga has a variant in Elizabeth Midford's elder brother Edward: he's not that much in favor of his sister's fiancé at first, but has slowly mellowed out in latter chapters...
Gauche from Black Clover is creepily obsessed with his ten-year-old sister, Marie. To the point where he tries to kill Asta in his sleep because Marie has a Precocious Crush on him.
In Brave10, Seikai is Isanami's adoptive older brother and will immediately resort to violence if he thinks anything untoward is happening. This is all the more stressful for Saizo as Isanami's very interested in him but he doesn't reciprocate, not that the distinction matters to Seikai.
Shu did that several times in Castle Town Dandelion:
- When Kanade jokingly claims that Fukushina was asking for Akane's hand in marriage in Chapter 6, he loudly expresses his objection.
- In Chapter 11, when Prince Alvin attempted to propose to Akane, Shuu flatly asked him to stop.
CLANNAD: Invoked by Tomoya in Mei's character arc. The two of them decide to pretend to be a couple (note: Tomoya is 17/18ish, Mei is 13) in order to awaken Sunohara's Big Brother Instinct and get him to prove that he still cares about his sister's well-being. It doesn't work, making Mei sad and Tomoya angry. It later turns out that Sunohara in fact trusts Tomoya to such an extent that he thought Mei would be all right with Tomoya as a boyfriend, even with the age difference taken into account.
Not surprisingly, Lelouch feels this way toward his little sister Nunnally. It's not really so apparent in the Code Geass series proper, but made much clearer in the Storybook Episode OVA Nunnally In Wonderland. The only exception to this rule is Suzaku who he is instead a Shipper with an Agenda about a potential relationship between him and Nunnally.
The Archaia retelling of Cyborg 009 has a rather disturbing variation of this trope. Kai isn't a friend of Joe's, but he's less than pleased to find out that Joe intends to run off with Kai's sister, Ayumi. The proper response to this is to use his connections to the mob to have Joe nearly beaten to death. After being turned into a cyborg, Joe tracks Kai down and tells him that he will let Ayumi live her life however she wants, or else there'll be trouble.
Death Note: When Touta Matsuda shows interest in Light's sister Sayu, who has grown into quite the cutie, no one is thrilled. Specially, Matsuda's boss Soichirou... her Boyfriend-Blocking Dad.
Komui Lee in D.Gray-Man has this bad regarding his little sister, exorcist Action Girl Lenalee Lee. At one point Inspector Link actually blackmails Allen by threatening to tell Komui that he is alone in a locked room with Lenalee. Allen immediately apologizes and opens the door. And when someone was ogling Lenalee, Komui's co-workers gave us this wonderful quote:
Co-worker: Section Chief Beck, I would advise you not to look at Lenalee that way.
Man: Why? Does she have a lover?
Co-worker: She has a brother.
In The Disastrous Life of Saiki K., Makoto Teruhashi is often maddenly protective of Kokomi, especially towards Saiki since he believes Saiki's in love with her as much as she does. Makoto also believes that Kokomi is in love with himself, so that plays a huge factor.
In Free!, Rin pulls Momo away when he starts hitting on Gou/Kou. He even outright tells Momo that his sister is off limits.
Subverted in Fullmetal Alchemist where Armstrong introduces his sister to Jean Havoc when the latter complained of not having a girlfriend.
In Great Teacher Onizuka, Kunio Murai is overprotective of his Absurdly Youthful Mother^noteOnly 13 years older than him, to the point of it being his Berserk Button whenever Onizuka shows interest, and jumping into a lake to interrupt their Old-Fashioned Rowboat Date, despite not knowing how to swim.
Inverted in Mobile Suit Gundam 00 with Michael Trinity getting angry at Setsuna when his younger sister Nena forcefully kisses the Gundam Meister, and getting even more pissed off when Setsuna pushes Nena away because he doesn't like the physical contact.
In Hetalia: Axis Powers, if you come out and tell Romano you're interested in his little brother he will flip the fuck out. Hell, even so much as barely indicate attraction.
Inverted in I Can't Understand What My Husband Is Saying, where Hajime eagerly gives his permission for Miki to date his little sister Mayotama. Of course, what he neglected to tell Miki was that Mayotama is actually his younger brother, and the entire reason he did so was because he was hoping that it would get rid of Mayotama's crush on him.
Kaguya-sama: Love Is War: Fujiwra is completely against the idea Shirogane dating her little sister Moeha because all the trauma from the times she's had to tutor him. That said, Shirogane isn't even aware that Moeha has a crush on him and almost certainly wouldn't be interested if he was as his feelings for Kaguya are practically at Single-Target Sexuality levels.
In the second episode of Kimagure Orange Road, Kyosuke and his pals (Those Two Guys) are ogling a leotard-clad aerobics class from a nearby restaurant. They soon see that Kyosuke's younger sisters are in the class, and sure enough, Kyosuke almost immediately covers the other boys' leering eyes.
- Later, in Episode 30, Kyosuke and Kurumi's twin-sister Manami sabotage Kurumi's first date.
Love Stage!! provides a rare male example. Izumi's doting older brother Shougo isn't at all thrilled that Ryouma is romantically attracted to Izumi. Shougo once challenged Ryouma to fight for Izumi's heart and always pushes him away if he gets too close to his brother.
In Magi: Labyrinth of Magic, Hakuryuu does NOT approve of the friendly relationship between Hakuei and Kouen.
Mametarou from Massugu ni Ikou is a dog but is otherwise this way towards his owner, Iku. He detests her boyfriend.
Towa Sakura in Monthly Girls' Nozaki-kun is curious to meet Nozaki, the guy his sister Chiyo is obsessed over but then tries to warn his sister via text that he's a pervert (thanks to misunderstanding why Nozaki is so interested in his tennis uniform). Sakura's friends mistake it for him being clingy to his sister and jealous of her crush. They reply on her behalf that "your sister is a pervert too", and he immediately backs off.
Frey in Mythical Detective Loki Ragnarok is like this when Freya is around Loki. What he doesn't realize is Freya is doing all the flirting.
Neji Hyuga from Naruto doesn't really show this in the main universe, but in the spinoff Rock Lee's Springtime of Youth, he constantly kicks and sends Naruto flying every time he comes near his cousin Hinata. He also furiously tells her that he would never accept her and Naruto together, because it's an infamy to him. When Tenten and Lee suggest that Gaara might have a crush on Hinata, he viciously replies with "over my dead body!" Still, it may not be just the Big Brother Instinct here, since, in this mini-series, Neji is more than once implied to have some sort of romantic affection for Hinata. Turns into Irony when Word of God confirms that his death actually brought Naruto and Hinata closer together after the War, and they even named their firstborn in honor of him.
DO NOT get any ideas of dating Noa's brother in Oniichan Control. SHE WILL KILL YOU. This isn't because she wants to protect him, of course, but because she wants him for herself. (Okay, maybe a little of both, but mostly the latter.)
In Rave Master, Haru does this trope when he learns of the relationship between his sister Cattleya and Shuda, getting really pissed off when he sees the guy's memories and learns they had sex. He did this earlier when, another guy was dating Cattleya and broke up her heart. So naturally, when he saw said guy again, he made every effort to punch him out again. Not to mention Shuda himself when finding out about that guy, he wanted to chop him up to pieces also.
In Secret, the usually comedic trope gets Played for Horror to its logical extreme. Shinichi Mitomo, the Big Bad, masterminded the murders of the students, turning them against each other, because he was pissed that someone impregnated his sister before he could and decided to take down the whole class to get at one guy.
In The Seven Deadly Sins, King becomes upset when he finds out that his sister Elaine and Ban had feelings for each other. Elaine is dead, but Ban plans to find a way to bring her back so that they can be together. King protests the "be together" part.
In SPY×FAMILY, Yuri zigzags this as it was by mostly his urging that Yor settle down that she looked for a fake boyfriend, but he becomes mad with jealousy when he learns that Yor has been married to Loid for over a year (which is actually a lie on the couple's part).
In Tales of Wedding Rings, Prince Jade is against letting Satou marry his younger sister Nefritis, partially because he believes he's honoring their late parents' wishes by protecting her from the outside world, partially because he wants to keep Nefritis for himself. This is despite the fact that Nefritis, as one of the five ring princesses, needs to marry Satou and help him save the world. He goes so far as to have Satou thrown in a dungeon, ostensibly to wait until Nefritis is of a reasonable age to marry... and since Nefritis is a Long-Lived elf, Satou will either be dead or a shriveled old man by then.
The Yu-Gi-Oh! anime has type four, or maybe type five with Jonouchi and his sister, Shizuka. This mostly affects Jonouchi's best friend, Honda, who is nuts about Shizuka (while Shizuka herself sees Honda as a friend).
In Yu-Gi-Oh! ZEXAL, Shark is type five as far as Rio is concerned. (Rio, however, isn't always happy about it.)
In Yu-Gi-Oh! ARC-V, it is implied that Shun was type 4 for Ruri. However, this is never elaborated due to the recent events concerning Ruri's love interest, Yuto.
- Due to it never being elaborated on in the anime, several fanworks seem to depict him as this.
In Yuri!!! on Ice, Michele Crispino has this attitude towards any man who so much as looks at his twin sister Sara, which stems from how boys used to bully Sara when they were younger. Unfortunately, Sara really wants to be able to date and finds her brother's behavior possessive and limiting.
YuYu Hakusho combines 3 and 4 with Yukina and her brother Hiei. Everyone who touches Yukina, especially Kuwabara, will get disemboweled and burned alive by Hiei.

Comic Books

Asterix: Carferrix's attitude to his sister Chipolata in "Asterix in Corsica". Hilarity Ensues when a Roman officer shows up at the door and demands to search the house. Carferrix orders Chipolata back indoors before she can even answer him, and proceeds to accuse the Roman of 1) talking to his sister, 2) not liking his sister, 3) liking his sister, getting progressively angrier until the officer runs for his life. For a moment, it seems he was just pretending to frighten the Roman away (since he was, in fact, sheltering three enemies of the state). He then turns around and scolds Chipolata for "flirting with Romans," clarifying that nope, he is a murderingly overprotective brother.
Family: Chrissie and Richie follow their sister Talia on their father's orders, then threaten Kurt (a black cop) for sleeping with her, though Talia intervenes to stop them. While it was initially a one-night stand, Talia and Kurt eventually grow closer together.
There's kind of a case for Hank Hall and Dawn Granger of Hawk and Dove. Dawn is the second Dove, the first being Hank's late brother Don, and he treats her like his sister. So when he suspects Deadman is trying to start a relationship with her, he makes it perfectly clear that she's off-limits. Doesn't stop her from starting something with Deadman, though.
Quicksilver is like this about his twin sister, the Scarlet Witch, up until she marries the Vision. It's typically got overtones of a Big Brother Instinct, but occasionally verges into... less platonic implications. And then there's Ultimate Quicksilver, who doesn't want anyone dating his sister because... well... yeah.
Spider-Girl is a rare female example of this trope, trying to keep her "little sister" April away from her manipulative Jerk Jock ex-boyfriend.
Superman:
- As far as Pre-Crisis Supergirl is concerned, her cousin Kal is off limits except for suitors previously approved by her. In Supergirl (1982), her friend Joan asks Linda whether she may introduce her cousin to her, and Linda makes clear that Clark is not available.
- Superman vs. Shazam!:
  - As they are working together, Supergirl grows quickly tired of hearing Mary Marvel gush over her cousin.
    Mary Marvel: But Superman is just so gorgeous... I hope he doesn't get hurt.
    Supergirl: (acidly) Let me worry about cousin Kal... Okay, Mary?
  - Eventually, Supergirl loses patience with Mary fawning all over Superman, so she decides to give her a taste of her own medicine by giving Mary's brother Captain Marvel a hug and an eyelash-batting. Suddenly, sweet-as-pie Mary blows off.
    Mary Marvel: That's my brother you've got your hands all over, Supergirl! Why don't you pick on someone your own age?
    Captain Marvel: Actually, Mary, I thought it—
    Mary Marvel: I DON'T CARE WHAT YOU THOUGHT!
Falling under the "extended family" category, Wonder Woman does not approve of Superboy's relationship with Wonder Girl. Demonstrated by the time the latter two were kissing in midair and Wonder Woman grabbed Superboy by the collar and flung him into a mountain range.
In the X-Wing Rogue Squadron comics, Wedge Antilles hasn't seen his sister since he was eight years old, and in that time she's actually become a famous holofilm star and married the best pilot in the Empire. Said best pilot in the Empire defects with the caveat that Wedge help him find and save Syal, who went into hiding. Wedge goes to some of his pilots and tells them that they're going to go and save Fel's wife, the actress Wynssa Starflare, who is also Wedge's sister; he seems okay with her being Happily Married to Baron-Colonel Soontir Fel, but this trope comes up anyway.
Hobbie: Why didn't you let us know you had a babe for a sister?
Plourr: (smacks him) Idiot, you don't ask that kind of question.

Fan Works

In the Mob Psycho 100 fanfic Everyone Love Mob by Green_Kneesocks Ritsu chases away everyone who even thinks about developing feelings for (gender swapped) Mob. Even if those feelings are mutual.
In the Death Note AU Addiction to Light's dismay Sayu begins flirting with Near.
In Chapter 43 of BlazBlue Alternative: Remnant, when Yang sees Ruby with Penny and briefly assumes they're on a date, she immediately enters what Ruby calls "Protective Big Sister Mode" and tries to intimidate Penny away. Though she drops this after spending a conversation getting to know Penny and becomes a Shipper on Deck for the two.
Gender Flipped in the Death Note AU Seigikan when Light notes that his sister Sayu gets scary protective of him when he admits the (technical) truth that he got beat up by one of his jilted lovers.
In the The Hunger Games fanfic The Bombshell by Jael.Rice.1., Kelvin, one of District 5's Tributes, remarks how Harvey, a friend of his, best keep his hands off his sister Curie on the grounds that "it's not what friends do to each other."
In Supergirl fanfic Hellsister Trilogy, Superman's first reaction when he finds out about his cousin dating Dev-Em is beating the latter up. Since pummeling Dev pisses Kara off, though, Kal-El has no option but to grind his teeth and accept their relationship.
Inverted in The Life and Times of a Winning Pony . Applejack runs the protagonist Cloud Kicker off her farm when she catches her talking to her older brother, Big Mac.
Also Inverted in Friendship Is Magic, as Applejack doesn't like Twilight Sparkle having a crush on Big Mac.
In If Them's the Rules Tom does not like the idea of Harry being friends with Madeleine or that creep Arcturus.
In Hogyoku ex Machina, Ichigo gets really angry when he learns of his sister Karin's past attraction to Hitsugaya.
In Mega Man Recut, Proto Man establishes this amongst the Robot Masters regarding Roll. He later gets angry when he learns that she might have a crush on Enker.
Stallions Of Harmony Verse: Gender and age inverted. Twilight does not appreciate Moondancer fantasizing about and ogling her older brother Shining Armor.
Twilight Sparkle: Stop fantasizing about my brother!
Age inverted in Ninja of Santoryu where Gaara threatens Shikamaru multiple times after he begins dating Temari, even threatening to mail him back to Konoha in a cardboard box.
Gender Flipped with the one being off limits the younger brother in the Tales of the Abyss Reflections. Asch is not entirely pleased about Tear's interest in his "twin" Luke. Downplayed in the end, as he ultimately accepts their relationship, but he's still obviously worried about Tear inadvertently hurting Luke.
A sidestory of Pokémon Reset Bloodlines reveals that Ash had a crush on Gary's older sister Daisy, much to Gary's dismay. While Ash eventually got over it, Gary still doesn't like him being close to Daisy.
Fates Collide, Jaune Arc gets angry when his sister Jeanne and Edmond Dantes have a chat. Jaune confronts him and orders him to stop flirting with his sister. Edmond is incredibly shocked and protests that they were just talking, but Jaune clearly doesn't believe him.
Fate Crazy Knights: Saber does not approve of Diarmuid dating Saber Lion because she thinks he's a pervert for having a Love Spell. Gawain also tells Diarmuid to keep away from Gareth.
Teen Titans Our Own League series:
- Similar to canon, Wonder Woman is extremely protective of her adopted younger sister, Donna Troy. When she notices Conner making heart eyes at Donna, she wastes no time in laying down the law:
  Diana: My sister is a pure, unspoiled refugee from the World of Man, not a pretty flower for you to pluck and cast aside! If you give me reason to believe you plan to dishonor her, may fire from Olympus strike me down, I will reach down your throat and rip out your spine!
- Conner gets warned off from Damian Wayne's half-sister Helena as well. Since Helena is just a baby, Conner is more befuddled than threatened.
This fanart reveals the real reason in Wonder Woman never hit on Supergirl's cousin in the Pre-Crisis DC Universe.
In The Institute Saga, Clark makes quickly known how he feels about people perving on his young cousin.
"Deeeyaaaaamn, she's sexy." Roberto half-whispered. "Who do you think she is?"
"That's a nice outfit, I wonder if I can talk her out of it?" Ray snickered.
"I'd cut down a bit on the comments." Spyke advised as he walked up to stand beside the group of admiring teens. "Clark has super-hearing, remember? He can hear every word you've said. Heck, he can hear your heartbeats from here."
Ray paled. "Erm, Clark, I didn't mean it! It was a joke! Honest!"
Clark turned his head slightly and the grass at Ray's feet ignited, only to be put out by Bobby Drake.
"I think that was a warning to think before speaking." Spyke noted, seeing Clark nod his head in confirmation.
Subverted in Flame's Shade. Yang (who herself had a short relationship with Blake in the past) has no issue with Blake dating her sister Ruby. She notes that she trusts Blake too much to give her the old "hurt my sister and I'll kill you" spiel.
Similarly, Yang also subverts this in Compromised Composure when she informs Weiss about Ruby's feelings for him. She explains it's better for Ruby to experience making mistakes while Yang is nearby to help her. She also points out that if Weiss hurts her, he'll end up hurting himself more than Yang ever could.
The plot of BRAVE TWINS revolves around Merida being unwilling to let her twin sister Erin be married off yet. She decides to take part in the games and win so that Erin won't be shipped away.
In Code Geass: Lelouch of Britannia, Cornelia is unnerved that her younger half-brother Lelouch is a Chick Magnet.
Fallout: Equestria: A soldier dying on a battlefield tells his sergeant his only regret is never meeting the sergeant's hot sister. The sergeant says now he has to survive, so that he can get his ass kicked for that later. The soldier was Steelhooves, and he did survive, and eventually ended up in a relationship with Applejack after her brother Big Macintosh died.
In TheNight130's video Demons, Jim is Cinderella's younger brother but he doesn't agree with his sister's relationship with Esmeralda. Jim finds a way to break them up so that they're literally unable to reunite.
In X-Men fanfiction Devil's Diary, Mastermind lusts after Wanda, but he's too frightened of her brother Quicksilver to try to seduce her.
Magneto's diary entry: The Toad thinks of himself as Wanda's defender. Wyngarde merely wants to bed her. And he knows full well that Quicksilver would dismember him if he did.
In Ben 10: Omniharem 10, Rook is not happy when his sister Shar starts flirting with Ben.
One Girl with Ten Brothers: Linka's brothers don't like it whenever she's hanging out with a boy, especially if the boy in question is her former bully Robbie Dan.
In the Harry Potter fanfic "This Means War", Ginny's six brothers scare off her boyfriend because they've decided she's too young to date. She is pissed, and decides to accelerate her plans to snag Harry so that it will be the two of them against her six brothers. Hermione, horrified, tells Ron that he is really going to regret fighting both of them at once.
Ginny: I was planning on doing this at the end of term, but you have forced me to move my plans forward. I will have a new boyfriend shortly, my darling brother. I will be finding closets to kiss him in. I will even happily let him find out what is under these robes, and there will be nothing you can do about it. But that's only the start. It will be then that we, and I mean my new boyfriend and I, will start to get revenge on you, then the twins, then Percy, then Charlie, and finally Bill. Every one of you will pay.
Ron: No one will go out with you. I've already put the word around that any one who dares will receive the full wrath of the six of us. I might not be that intimidating on my own, but with the others, the boys all understand.
Ginny: I've been planning this for a long time, far longer than you can imagine.
In "Seven", when Daniel Jackson (Stargate SG-1) is revealed to be Humanoid Cylon Model Seven (Battlestar Galactica (2003)), once he returns to the Twelve Colonies and makes contact with the would-be Caprica Six, he immediately expresses disapproval of her relationship with Gaius Baltar.
Son of the Sannin: Sasuke becomes a bit displeased when Itachi begins a relationship with Natsu Hyuga, after he saved her during Akatsuki's invasion, partly because they're no longer able to spend as much time together as before. Both Shizune and Sakura rib him over it and Natsu herself takes it in stride.
In the Resident Evil 2 fanfic Breakaway, Chris Redfield's default reaction to any guy trying to make a move at his baby sister is to "take them out back to the courtyard and show them what’s what before they get any ideas". Claire never has any boyfriend because of him, and the only reason Leon managed to get a date with her is because he managed to ask her out before he knew she was Chris's sister.
In the Code Geass fanfic The Japanese Are Already Dead, Lelouch is this to his sisters Cornelia and Euphemia, something he can back up thanks to becoming the Emperor of Britannia. When he discovers that Guilford has Cornelia's handkerchief, he quickly grasps the implications of the two being together and arrests Guilford before attempting to have him dishonorably discharged of his rank until Cornelia knocks him out. And when Suzaku tells Lelouch of his plan to propose to Euphie after years of dating, Lelouch pins him to the wall and tries to recruit Cornelia's help in getting Euphie away before the more fit knight knocks him out.
Gender-flipped in Everyday Life with Ultimate Girls, with Komaru Naegi wanting to see if any of her big brother Makoto's dormmates in the girl's dorms (due to his Gender-Blender Name and the fact that the intended Ultimate Lucky Student was a girl, he ended up in the girl's dorm, and the luck of Nagito left the remaining boy's dorms in a wreck, meaning he had no choice but to stay there) are good enough to date him. Thus far, this has only played a part in Aoi Asahina's chapter, where the three are also accompanied by Yuta Asahina. Komaru does note her Nice Girl nature, and likes that Aoi knows about Makoto's similar personality, and even realizes that the Ultimate Swimming Pro has begun to develop a crush on Makoto.
Superman in The Ghost Boy And The Super Girl is none too happy when he sees how close Kara and Danny are despite having just recently met.
Fire Emblem: Three Houses: Fifth Path features a version of this with Byleth and her father, Jeralt. Specifically, Byleth gets rather miffed when Manuela starts flirting with her father.
Guiding Light (AuroraRose2081): While on her first walk in town, one of Camilo's friends, Lorenzo, becomes quite flustered around Mirabel. Camilo promptly scares him away using Augustín's image.
Completely subverted in Skyhold Academy Yearbook. Varric thinks that this is how Hawke will react if she finds out that he's secretly in love with her sister. As it turns out, not only has she long since figured it out, but she couldn't be more okay with it.
In the Love Live! Sunshine!! fanfic The D in Dia stands for Death, Dia Kurosawa is very protective of her little sister Ruby and wants to avoid her being taken by a heathen. This becomes a problem for Ruby as she is dating Hanamura and they worry Dia would hurt her despite all them being in the same idol units. Thankfully, Dia Subverts this trope and is instead relieved that Ruby is dating Hanamura instead of anyone else because she knows how much Hanamura cares for her sister.
𝕸𝖞 𝕯𝖊𝖊𝖕𝖊𝖘𝖙, 𝕯𝖆𝖗𝖐𝖊𝖘𝖙 𝕾𝖊𝖈𝖗𝖊𝖙: Well, brother, actually. It's mentioned that the vampires often scare off any admirers of Solon. Solon recalls a time seeing Jino, of all people, burn a bunch of love letters meant for him during Valentine's Day. Solon didn't even meet the wolves until the time of the story because a few years ago, they stared at him for a few seconds and they thought they were ogling him. Kind of justified, given that the people Solon tends to attract can either be a good or bad person, and the complicated nature of people makes it hard for them to tell which is which. Luckily, aside from intense interrogating, Enzy didn't suffer when they found about his and Solon's relationship, and they even let them stay together.
- Interestingly enough, they were happy when it seemed like Solon got a love letter (really just a note from Enzy telling him to meet up somewhere) and Shion often teases him about having a boyfriend. Though since they were with the Nightball team at the time, it's likely they were just teasing him outside then planned on hunting the one who sent the letter later on, and Shion actually seemed happy Solon was with someone who loves him.
The Dragon's Son: Kagami scares off anyone that tries to asks out her cousin Izuku, and says they would have to beat her in a duel if they want to date him. Justified as Izuku is embarrassed that strangers are trying to date him and is too awkward to say no, so he's grateful that Kagami is turning them down on his behalf.
In The Pirate's Soldier, Misao Kuramitsu is none too happy when Heero Yuy gets engaged to his sister Mihoshi to seal the alliance between the Jurai Empire and the Seniwa Domain. He actually decides to go behind his family's back and contact Kagato as a means to prevent the marriage from going through.

Film

All the Real Girls: The friendship of Paul and Tip is tested when Paul falls for Tip's younger sister Noel.
30 Minutes or Less, among other things, has the protagonist reveal to his best friend that he has slept with his twin sister. The friend is pissed, as he reasons that sleeping with one twin is the same as sleeping with the other.
Downplayed in Blackhat: Chen is a Chinese Army officer, his sister Lien is a network engineer, and Hathaway is an American hacker who has been let out of jail to help them track down a cyber-saboteur. Chen does not try to stop Hathaway sleeping with Lien, but he does tell Hathaway that he isn't much of a catch, even if he doesn't have to go back to jail.
Boy Eats Girl: Girl Posse member Charlotte is surprised and hurt when she finds a pair of underwear that she thinks belongs to her sister in her Really Gets Around boyfriend Kenneth's car. He denies it, pointing out her sister is only fourteen, but a coy smile he makes as he turns his head away implies he was lying.
CODA: Ruby’s best friend Gertie has an attraction to Ruby’s older brother Leo. Even though Ruby is a little squicked out by her friend’s crush, she doesn’t object to them becoming a couple.
Gender-flipped example in Crimson Peak, where Lucille Sharpe is not at all happy at any hint that her brother Thomas might be involved with a woman. This is partially protectiveness, since she grew up shielding him from their abusive parents' wrath, but also due to the sexual/romantic relationship they've had since adolescence. She also has homicidal tendencies.
In Face/Off, this is the source of Dubov's anger at Troy. Later inverted when Archer as Troy recruits Dubov to help him escape by telling him he didn't actually do it.
In the first The Fast and the Furious Brian has a serious crush on Dom's sister Mia. Dom is not pleased.
Subverted in the TV movie of Anne Rice's The Feast Of All Saints. Richard very formally asks his best friend, Marcel, for permission to court Marcel's younger sister, Marie. Marcel bursts out laughing at the awkward formality of the situation, then immediately gives his permission since "You're a lot bigger than me, you know!"
The brothers in Fools Rush In seem to be like this, or at least that's what Alex fears.
Giant Little Ones: Ballas is outraged after Franky and his sister Natasha start dating, claiming it was just to prove Franky's straight (but this isn't the case). They both roundly ignore his injunctions not to see each other.
G Men: Jeff McCord does this to Brick in regards to his sister Kay, at least until Brick rescues Kay from the Big Bad.
A minor example of this trope from Groundhog Day, when old classmate of cynical Phil Connors bumps into him in Punxsutawney, PA, and is trying to get him to remember him:
Ned: Ned... Ryerson! "Needlenose Ned"? "Ned the Head"? C'mon, buddy. Case Western High. Ned Ryerson: I did the whistling belly-button trick at the high school talent show? Bing! Ned Ryerson: got the shingles real bad senior year, almost didn't graduate? Bing, again. Ned Ryerson: I dated your sister Mary Pat a couple times until you told me not to anymore? Well?
Phil: Ned Ryerson?
Ned: Bing!
Phil: Bing.
Hobbs & Shaw: Deckard Shaw becomes upset when he notices Luke Hobbs is attracted to Shaw's sister Hattie and tells him he had better not try anything, but Hobbs taunts him by saying soon he will have "horizontal hula sexy-sexy time" with his sister.
A major factor in Living Will, with Belcher upset about Will and Krista's relationship.
In The Loft, Philip is violently protective of his kid sister Zoe. He almost assaults Marty for making lewd comments about her, and after learning that Vincent has slept with her, he stages the crime scene to make it appear more violent, and inadvertently causes Sarah's death.
In Most Likely to Murder (2018), Duane repeatedly tells Billy not to go near his younger sister Tami. When Billy tries to kiss Tami under the mistaken impression that she's interested, Duane is enraged and temporarily ends their friendship.
Scarface (1983) does this to a tragic level. Tony Montana is paranoid that his sister, Gina, would enter his criminal lifestyle. When he finds out that his friend Manny Ribera had been seeing her, he shoots Manny dead without a word. Gina then reveals that the two of them had just been married. Gina later lampshades the somewhat Freudian motivations behind Tony's overprotective attitude towards her, coming on to him (mockingly) just before trying to shoot him. The original 1932 version has this as well; both are a type 6.
Secretly, Greatly: Yoo Joon is introduced slapping Dong-gu for mooning after his sister Yoo Ran.
The 1998 Donnie Yen kung-fu film, Shanghai Affars, stars Yen as an apprentice doctor whose Love Interest is the little sister of the local axe gang's boss, who strongly opposes their relationship and repeatedly tries to have Yen kicked out of the town. To complicate things even further, the axe gang leader is in cahoots with Yen's mentor in an Organ Theft business.
In Shanghai Knights, Chong Wang initially objects very strongly to a relationship between Roy O'Bannon and his sister Lin, and in an reversal of a scene from the first movie, Roy overhears him talking badly to her about him. (He does have some reason to worry Roy wouldn't be good to her.) Eventually he comes round, though he still tells him "Break her heart, I break your legs" which Roy accepts as fair.
An all-female and probably non-romantic (despite a storm of Les Yay) variation takes place in Sucker Punch: Rocket is obviously fascinated with Baby Doll and would follow her into a reckless escape attempt. Rocket's Cool Big Sis Sweet Pea (who has a bad case of Big Sister Instinct towards her) is very opposed to them doing anything together and only lets them carry on with her keeping an eye on them. Turns out, she was right: Baby Doll's plan does get Rocket killed.
A Film Noir called Sweet Smell of Success takes this to a dark extent, in which a reporter ruins the boyfriend's life in order to get him away from his sister.
Played for laughs in Tango and Cash where Tango doesn't want rival detective Cash dating his sister Katherine, after apparently catching them doing something intimate on the couch—she was actually massaging a slipped disc back into place, but neither Cash nor Katherine bothers to explain this. When Cash says he'll wait until he has Tango's blessing before dating her, he tells him, "In that case, never!"
The Dutch short film "Turkse Chick" features a skateboarding teenager finding himself attracted to a young Turkish woman in the park, who then spontaneously takes him into the bushes for a tryst. He's obviously dumbfounded, but before anything can happen, her brothers appear to defend her honor, one of them blowing him up in a Suicide Attack. Turns out it was All Just a Dream.
The Unforgiven although they become romantically involved later in the movie This is the spirit of the thing in the scene where Johnny Portugal rides up to Ben Zachary's step sister Rachael.
Johnny: You have a burr in your pretty hair.
[Ben proceeds to rip Johnny off his horse, punch him and they stop short before brawling. After things cool down, Ben shouts to everyone present]
Ben: Anybody else see a burr in my sister's hair?
In Written on the Wind, Kyle beats up a man to keep him from sleeping with Kyle's sister Marylee.
In The Angry Birds Movie 2, Chuck makes it very clear that his little sister, Silver, is off-limits, threatening to break every bone in Red's body if tries anything. By the end of the movie, he seems to come around to it, though.
Young Hearts: Adam, Tilly's best friend, starts to shun him after Tilly has gotten involved with Harper, Adam's sister (and they lost their virginities together). Tilly is saddened by this.

Literature

In Accel World, Ash Roller isn't fond of the idea of his Friendly Rival Haru/Silver Crow getting together with his little sister Rin (who, as a result of complicated circumstances, technically is Ash Roller; she had Brain Burst installed on her brother's Neurolinker). That said, when it looks like Ash might have to be erased from Brain Burst forever to protect Rin from the ISS kit parasitizing Ash's bike, Ash encourages Haru to be friends with his sister, so long as they're just friends.
In one Animorphs book set in another timeline, Marco wondered if his best friend Jake would mind if he dated his cousin Rachel, and concluded that she was hot enough that he didn't really care. They hook up in that reality, though in the normal timeline Marco only ever flirts with her while she instead dates Tobias. In either case, Jake doesn't seem to care.
The only redeeming feature of Dallas Genoard in Baccano! is his violent reaction to his friends (who, like Dallas himself, are two-bit thugs) ever mentioning his little sister Eve.
Beaufort by Ron Leshem. One of the soldiers is embarrassed because his best friend had started dating his sister and insists on telling the entire barracks just how good she is in bed.
In the Brotherband book Slaves of Socorro, Thorn and Karina go to a village festival together (Karina insists it's as friends). The next morning, Thorn nervously has a talk with Hal, assuring him that no funny business occurred between him and Hal's mother. Hal finds the incongruity of the situation amusing and reassures Thorn that if he and Karina want to court, it's completely fine with him.
Due to a scarcity of men, it is generally the brothers that are off limits in A Brother's Price. One of the protagonists severely damages her friendship with the main protagonist's sister when she seduces him. It gets better when she makes it clear that she fully intends to marry him.
In the original Dragonlance trilogy, boyhood best friends Tanis and Gilthanas fell out over the former's romance with Gilthanas's younger sister. Tanis is a half-breed and a bastard, while Gilthanas and his sister Laurana are a royal prince and princess, and thus Tanis is considered unworthy of her.
In Peter Tinniswood's novel Except you're A Bird, the central character, Carter Brandon, is befriended by local hero, professional rugby player Artie Shirtcliffe. Carter is drawn deeper into the Shirtcliffe family, and the deeper he goes, the more clues appear as to something being not right. His wife Pat being in hospital, Carter develops a dangerous obsession with Artie's sister Alison. Warned off by Artie, he suspects this is because he is otherwise happily married. Eventually he discovers the awful truth: the Shirtcliffe siblings are in an incestuous relationship. After he discovers them in flagrante, there is a double suicide attempt.
- There are also hints in the book that Carter and his mother might have had an unhealthily close relationship that hovered on the brink of incest, without having ever got there. The revelation about the Shirtcliffe siblings certainly pushes Carter over the brink into an emotional crisis.
This attitude is the death of Margarita's brother in many versions of the Faust legend.
Inverted in Harry Potter. The eponymous protagonist was really afraid that his best friend Ron would react like this when he started falling for his sister, Ginny. Instead, after their first and very public kiss in Half-Blood Prince, his reaction was basically "fine, if you have to". In the previous book, Ron finds out that Ginny is dating a Ravenclaw boy and his first reaction is "but she's supposed to have a crush on Harry!" (she had in the second book). Later, when Ginny and the Ravenclaw break up, Ron suggests she choose "someone better" next time and gives Harry a look that he fails to understand. (It may help that he's something of a celebrity, a decent guy, and a friend to Ron.)
Inverted in High School D×D. Sirzechs is perhaps the most enthusiastic Issei/Rias shipper of the lot, in a series where even her eager harem-mates want to see them hook up.
A reversed example in Infinite Stratos: Chifuyu Orimura, possibly the world's strongest IS pilot, is very much aware of the Unwanted Harem that pines for her younger brother Ichika, but she makes it clear she's not simply letting them just have their way with him. To be fair, she sees Ichika as being her only family. She even has a 'woman-to-woman' talk about him with 5 of the girls. She later regrets this in the OVA when she talks to Yamada-sensei about it since she realizes that because of the way she said it — she had a few beers during the talk as well — the girls think she's yet another rival for her brother's affections.
The Ink Black Heart: Racist youtuber Wally Cardew's one Bigotry Exception friendship comes to an end after he finds out that his Middle-Eastern friend is in a relationship with his sister, leading to a fight between the two of them.
Averted with Renzi and Kydd, who actually helps his friend to try and impress his sister, Cecelia.
Journey to Chaos: Forge Enaz takes an immediate dislike to Eric simply because Eric is dating his twin sister.
Les Enfants de Prométhée: Dimitri is initially very opposed to Eric wooing his sister Olympe, mostly because he and Eric hate each other, while Olympe is the only person who loves Dimitri in spite of his Hair-Trigger Temper and the superstitions surrounding his origins. However, when Dimitri falls deathly ill, he resolves to set on a journey to find Eric and make sure that his sister won't be alone after his death.
My Next Life as a Villainess: All Routes Lead to Doom!:
- Keith is an unusual example. He regularly butts heads with his step-sister Catarina's fiancé Geordo and is working to have the engagement called off. The reason however is because he is in love with Catarina and wants to marry her himself.
- Alan considers his fiancée Mary to be like his little sister and confronted Catarina over her supposed seduction of Mary. This however goes away completely after Alan falls in love with Catarina.
Oreimo:
- Both Kyousuke and his friend Kouhei more or less tell each other they're not okay with anyone dating their respective sisters. In Kyousuke's case, it's likely because he feels Kirino's not old enough to date yet (she's 14), and in Kouhei's case, he's got hints of Brother–Sister Incest with his sister Sena.
- Inverted example with Kyousuke's sister Kirino, who gets extremely jealous anytime she sees him hanging out with a woman other than herself, especially his Childhood Friend Manami. It gets to the point where she brings home a boyfriend in volume 7 of the light novel, and lets him experience how she feels whenever she sees him with his female friends. It turns out that the boyfriend was just faking it along with her to go along with her plan.
A somewhat unusual version occurs in The Outsiders. Ponyboy comments that Dally has no qualms about swearing, smoking, or talking dirty to a greaser girl; the rest of the gang even joins him sometimes. But they all — including Dally — act more polite to "the cousinly type", which includes cousins, sisters, and girls they see often at school.
Patience and Sarah: Sarah's teenage sister Rachel becomes jealous of Sarah's bond with Patience. When Sarah says that she loves Patience differently from Rachel and that she wants to kiss Patience, Rachel innocently counters that she could kiss her and do whatever Patience could. When Sarah refuses to take Rachel with her when she moves with Patience, Rachel ends up telling their father about Sarah's attraction to Patience, causing him to try to beat her into not loving Patience.
The Pet Girl of Sakurasou:
- Inverted with Jin and Fuuka. He started dating her because he didn't feel he was worthy of being with her sister, Misaki, the one he really loved. Fuuka realizes this and breaks up with him, telling him to man up and confess his feelings.
- Played straight (though gender and age inverted) with Yuuko, Sorata's annoying little sister. She has a Big Brother Attraction and becomes a Clingy Jealous Girl when he's around his female friends.
A lot of the Will They or Won't They? tension in the The Princess Diaries books has to do with the main character's crush being her best friend's brother. When they get together, everything's fine. But when they break up...
Rare non-romantic variant in Solar Defenders: The Role of a Shield: Dave is absolutely opposed to his sister joining the team and makes it clear that he won't stand for her being put in danger like that.
In William Faulkner's The Sound and the Fury, Quentin Compson massively fails to "protect" his sister from the many, many men she's having sex with, despite the fact that he is so desperate to keep her off limits. He loved "some concept of Compson honor precariously and (he knew well) only temporarily supported by the minute fragile membrane of her maidenhead as a miniature replica of all the whole vast globy earth may be poised on the nose of a trained seal. Who loved not the idea of the incest which he would not commit, but some Presbyterian concept of its eternal punishment: he, not God, could by that means cast himself and his sister both into hell, where he could guard her forever and keep her forevermore intact amid the eternal fires."
Warrior Cats includes a rare sister variation. Ivypool is strongly opposed to Tigerheart's advances towards her sister Dovewing, although this mostly has to do with the political and social dynamics of the situation, as well as their rocky history. She's determined to protect her sister from the humiliation and heartbreak such a relationship would bring.
Jeremy Agriche of The Way to Protect the Female Lead's Older Brother is extremely possessive over his older half-sister Roxana, to the point when he tries to kill Cassis Pedelian more than once after Roxana asks their father for Cassis as her (implied) Sex Slave. Later he attacks other people for just looking at her or mentioning her name.
Words of Radiance (second book of The Stormlight Archive): When Adolin sees a woman he had once courted, he tries to remember why they broke up, and asks towards her sister. She curtly responds that she's still off-limits.
Adolin: [thinking] Oh, right. That was what he'd done. Honest mistake.

Live-Action TV

Bellamy Blake on The 100 is very over-protective of his sister Octavia, since he's had to protect her her whole life. This continues when they're both sent down to Earth, and since she often disobeys him, he has one of his goons babysit her. Octavia flirts with said babysitter and before you know it, they're making out in the forest. Bellamy discovers them before it goes further. His reaction to this is to hang the guy up in the middle of the forest and leave him there for a while.
Arrow. At the start of the series, Oliver Queen returns home to find his kid sister Thea is now a seventeen-year old Hard-Drinking Party Girl, leading to a Running Gag of Oliver taking an instant dislike to any male she shows an interest in or vice versa. He doesn't even make an exception for his best friend Tommy Merlyn, which is just as well given The Reveal in Season 2 that Thea is Tommy's half-sister, via their father Malcolm Merlyn.
Tommy: Have you noticed how hot your sister's gotten?
Oliver: [Death Glare]
Tommy: Because I have not!
- Tommy just can't catch a break with this trope.
  Tommy: Girl's pretty cute.
  Diggle: She's my sister-in-law.
  Tommy: Who I will never speak to... or look at... ever.
- When Cisco from The Flash (2014) comes to visit Starling, he meets Thea.
  Cisco: Yo, who was that hottie?
  Felicity: [simultaneously] Oliver's younger sister.
  Roy: [simultaneously] My ex-girlfriend.
  Cisco: So stay away, that's what you're saying...
Beverly Hills, 90210: Brandon doesn't like it when Brenda falls for Dylan, knowing of Dylan's reputation (he's an alcoholic, among other things). Indeed, when Dylan stands her up, Brandon tells him off and warns him not to hurt her. However, it's the only warning Dylan ever gets, as Brandon never intervenes nor ends their friendship over the numerous other times Dylan breaks Brenda's heart.
Played for Laughs in The Big Bang Theory: Raj tries to forbid his sister Priya from seeing Leonard, but they ignore him. Sheldon voices his disapproval of Leonard betraying a friend - not Raj, Howard, as Howard and Leonard previously made a verbal agreement not to go after Priya. Sheldon, however, doesn't seem to care whom his own sister Missy dates. Leonard briefly convinced Sheldon to put Missy off limits, which lasted until Missy found out that Sheldon was attempting to control her sex life and expressed her disapproval.
Big Time Rush: Kendall does not approve of Bobby's interest in Katie. It doesn't help that Bobby calls her "Shmoking hot" and tried to kiss her twice.
- James also opts this attitude for Katie (despite not being her brother) when a boy decides to take her out. Although he acts more like a Boyfriend-Blocking Dad than an overly concerned brother. He even presents a gun-like weapon when asking him about his intentions towards Katie.
Bridgerton: In season 1, Anthony, who is now Lord Bridgerton and in charge of his family's estate, has high standards for his sister and shoots down any man she shows interest in. He's less than pleased when Daphne and his old friend Simon, whom he knows to be a rake, start talking. Later in season 2, Kate Sharma is very protective of her younger sister, Edwina, and scrutinizes her potential prospects carefully — especially once Anthony starts courting her sister.
Subverted on Chicago P.D., as Voight tells Lindsay and Halstead to stay away from one another, not because he worries about his surrogate daughter, but because he's aware of her penchant for leaving men broken-hearted and doesn't want to risk Halstead losing focus on his police work.
Chilling Adventures of Sabrina: While Zelda Spellman seems to have had quite the exciting time back in her younger days, her brother Edward (understandably) drew the line at allowing his old friend Faustus Blackwood to propose to her. That conversation apparently went so badly that Zelda is genuinely shocked when Blackwood finally tells her it ever even took place.
Dexter: Without ever stating it out loud, Sergeant Batista heavily implies this to intern Louis when he starts dating Batista's little sister Jamie. It's portrayed as just being overly protective of her, but the boyfriend in question turns out to be a scumbag regardless. Subverted later on when Jamie starts a secret relationship with Batista's colleague Quinn. He's supportive of it when he finds out, but wants Quinn to man up by applying for the vacant Sergeant post as Batista himself is now Captain.
The titular characters of Drake & Josh get a little uncomfortable at the prospect of Megan kissing a boy. This is one of the few times where their Big Brother Instinct kicks in, especially since Megan has done nothing, but prank them for almost the entire show. It later turns out that boy Megan was dating was actually cheating on her, and when Drake and Josh proved it to her, she actually thanked them for helping her realize what a jerk he was.
Subverted exactly the same way in ER. Gallant clearly doesn't want his MS-afflicted sister becoming involved with Pratt, who is something of a player. However, when said sister gives Pratt the brush-off after their night of passion, it becomes obvious that Gallant was trying to protect Pratt from HER, not the other way around.
The Falcon and the Winter Soldier: When Bucky Barnes meets and starts flirting with Sam Wilson's sister Sarah, Sam tells him to cut it out or he'll have him cut up and fed to the fishes. Amused, Bucky keeps doing it.
The Family: Bridey gets involved with Danny, then his sister Willa as well. Danny is not happy when he finds out, mostly because he thinks Bridey is just using her to get information.
A bit of awkwardness ensues on Flashpoint when Spike briefly begins dating Sam Braddock's sister Natalie. Though in this case, he implies he's at least partially looking out for Spike too.
Spike: (Referring to Natalie) We're not dating.
Sam: Well, that's good because that would be weird. Trouble has a way of finding her.
Inverted in Fresh Off the Boat; Trent forbids Eddie from dating his sister, Tina, to protect Eddie from her, as he considers them a bad fit. Eddie dates her anyways and they hit it off... until Tina starts pushing Eddie to quit being a Lazy Bum.
The Fresh Prince of Bel-Air: Will and Carlton are this with Ashley, who is Carlton's sister and Will's cousin. Strangely, neither of them seem to care about who Hilary, Carlton and Ashley's older sister and Will's oldest cousin, dates or sleeps with — but to be fair, of the four of them, Hilary's the oldest while Ashley's the youngest (she was only around 12 or so when Will first moved in with them). Plus, it's implied that Hilary Really Gets Around — so combining that with the fact that Hilary's the oldest, Will and Carlton probably see no point in trying to "protect her" (at least not like they do with Ashley). In fact, Carlton once tried getting into a university by advising the university rep how to get Hilary into bed and also advised her to get out of problems with the IRS by wearing sexually provocative clothing.
Friends:
- Ross discovers that Chandler's now in a relationship with his sister Monica and angrily declares "I can't believe it, my best friend and my sister!". When Chandler explains that they're in love and not just fooling around, Ross repeats it, only this time overjoyed. Of course, he was taking anti-rage drugs at the time. Doubles as Hypocritical Humor, since Ross has been into Monica's best friend Rachel for a long time and had dated her by the Monica/Chandler hook up. Unlike Ross, Monica is happy for Ross and Rachel to hook up.
- Rachel plays this trope straight, yelling at Joey the second he tries to hit on her sister Jill, and immediately feeling uncomfortable when Ross ends up dating her briefly. The relationship doesn't last for long, however.
- In Season 3, Chandler drunkenly kisses one of Joey's sisters. Joey starts out angry but calms down when Chandler denies that he's using the sister to get over his recent break-up with Janice... only for Joey to get mad again when he learns Chandler doesn't actually know which of Joey's seven near-identical sisters he kissed.
- The first season has a variation when Joey starts dating Phoebe's identical twin sister Ursula. Phoebe is upset but not because she's worried about Ursula being hurt. She's afraid that when Ursula inevitably breaks Joey's heart he'll end their friendship because Phoebe will remind him too much of Ursula. Ultimately Ursula does dump Joey but his friendship with Phoebe remains intact.
- It was once stated by one of the guys that dating sister could be tolerated, but mothers were strictly off-limits.
- In Season 1, Chandler's mother and Ross impulsively kiss, causing Chandler to become furious with Ross. He says that he expects this kind of behavior from her, but at the end of the episode ends up Calling the Old Man Out for her constant carelessness and emotional neglect since he was a child. Not that his speech affects her in the slightest, but Chandler later states that he feels good that he at least got it out.
Subverted in Full House. Danny isn't at all happy with his sister dating Joey, but finally tells Joey that it's not cause he's worried about his sister, but that he's being protective of Joey. As her job requires a lot of travel, he didn't want Joey's heart to get broken.
Game of Thrones:
- After Loras Tyrell is bethrothed to Cersei Lannister, he is on the receiving end of this from Jaime Lannister. Of course, Jaime has unconventional reasons for making this threat. An unintimidated Loras snipes back at Jaime that he won't marry her, either.
- Just look at the Death Glare Robb gives Joffrey when he sees him making eyes at Sansa.
- When Jon Snow leaves Sansa in charge of the North while he meets with Daenerys in the Dragon Isles, he tells Petyr Baelish he'll kill him if he tries to make a move on her.
Ginny and Georgia: Max shuns Ginny, and so do her friends, when she discovers Ginny's having sex with her brother Marcus. Her main beef seems to be that Ginny didn't tell her about it. She thought they were both virgins at the same time.
On Growing Pains, a girl befriends Carol Seaver just so she can get close to Carol's brother, Mike. Carol feels hurt about being used.
- In another episode, Mike's friend Boner wants to take Carol to a dance. Mike is upset at first but later accepts the pairing.
- Yet another episode saw Mike coming home from school and explaining to his father that he beat up one of his friends. It seemed that his friend told Mike a rumor that Carol and her boyfriend were sleeping together. Mike didn't understand why he got so upset until his father explained this trope to him.
Coop's sister in Hannah Montana when she tries to date Jackson. Coop gives him the 'Don't make eyes at my sister' speech. Unfortunately since she's presented as a slut, it loses some of its effectiveness.
- Miley has a dream where her best friend, Lily, had feelings for her older brother. She does everything she can to prevent a possible relationship taking place. However, she learns by the end that if Lily likes Jackson, then she'll just have to accept it. Of course, this concept is also a dream and Lily is repulsed by the idea of being with Jackson.
Herman's Head. Herman doesn't like his best friend, Handsome Lech Jay, sleeping with his sister because he still thinks of her as young and innocent when She Is All Grown Up and just as lecherous as Jay. Lampshaded in another episode where the two are laughing over all the Wacky Hijinks Jay has gotten Herman involved in. "And (ha-ha) remember the time I slept with your sister?" Cue immediate Death Glare from Herman.
Subverted on Heroes. When Hiro's time travel inadvertently Set Right What Once Went Wrong, causing Ando and his sister to fall in love, Hiro is ecstatic.
In How I Met Your Mother, The Casanova Barney makes it clear ^note that he finds Ted's little sister attractive, and Ted tries to prevent him from meeting her. It doesn't work, but they don't sleep together. Barney gets his comeuppance big time in "Ring Up", when Ted actually sleeps with Barney's younger half-sister, whom he's apparently pretty protective of.
Defied in Impractical Jokers where Sal's punishment was to be strapped to a flat board, unable to do anything as his close friend Murr marries his sister. Everyone in the church cheered on the lovely couple... everyone but Sal.
Murr: I, James Murray, take you, Sal's sister, to be my lawfully wedded wife. I take you for who you are, and who you are is Sal's sister.
Jessica Jones is very protective of her best friend/adoptive sister, Trish Walker. Trish notes that Jessica hates all of Trish's boyfriends.
Most people from the small town of Letterkenny have this attitude, but especially Wayne with regards to his sister dating arsehole hockey players Jonesy and Riley.
Leverage: Cora, who owns the bar the team works above starting in season 2, is like Nate's niece. Eliot is not allowed to like his niece.
In The Middle, Sue dates Darren, who is one of Those Two Guys always hanging out with her brother Axl, for a few episodes. Axl doesn't take it well — not because he's protective of his sister or anything, but because he thinks she is the biggest loser ever to walk the face of the Earth. Although later on when Darren unintentionally hurts Sue, Axl lectures Darren and almost slams the door in his face.
In NCIS, DiNozzo asks why McGee didn't tell him that he had a little sister and Ziva rolls her eyes. Naturally, no one would tell Tony that they had a little sister...
In New Blood Stefan first meets Arresh after hitting on his sister. Arresh objects and isn't happy about the fact that working with Stefan means more opportunities for Stefan to flirt with Leila, complaining that Leila better not actually be interested in Stefan.
Northern Rescue: Subverted. Maddie is upset when she finds her friend Gwen flirting with and spending time with her brother Scout. But only because her dad had threatened to ground her if she had Gwen over, considering her to be a bad influence (which she had been, but just for one episode and wants to get Gwen out of their before their dad gets back, and actually doesn't mind Scout and Gwen hanging out in general.
Peaky Blinders has Ada Shelby dating Freddie Thorne in secret before he accidentally gets her pregnant, much to the disapproval of her brothers, especially Tommy, who does his best to get rid of Freddie. When confronted about this by Polly, Tommy clarifies that he suspects that Freddie (who is a Communist) is just using Ada to find out Peaky Blinders secrets. Eventually he does come around to the relationship when it becomes clear Freddie cares far more about Ada than he does his political leanings and reconciles with both Freddie and Ada by the end of the first season.
Subverted in Peep Show, in which Jez specifically begins dating Mark's sister precisely because it upsets Mark. While an act of spite to start off with, it soon backfires on Jez when it occurs to him exactly how similar Mark and his sister are — while they're having sex...
Power Rangers:
- Power Rangers in Space did this with the usually Ho Yay-tastic Andros and Zhane. Thing is, little sister Karone was the Big Bad Astronema at the time, nobody knew she was Andros' sister, and the episode where they'd dated was very crack-tastic. It was more of a humor-oriented callback to that episode than actual disapproval. The best part was that they had the argument while sneaking into the enemy base to rescue her.
  Andros: Let me get this straight. You went out on a date with my sister?
  Zhane: I didn't know she was your sister!
- In Power Rangers RPM, Gem seems to disapprove of the potential relationship between his twin sister, Gemma, and Flynn. Of course, this has less to do with his opinion of Flynn, and more to do with the fact that, prior to Gemma's decision to help Flynn with his work, they had been Single-Minded Twins; this romantic initiative on her part meant that he'd have to start forming independent thoughts and sentences.
- Brought up for all of five seconds after the reveal about Tenaya and Dillon (aka Karone and Andros 2.0). When Ziggy notices that Tenaya's kind of hot when she isn't trying to murder them all, Dillon is quick to cut him off.
Gus on Psych has a Big Brother Instinct for his older sister Joy. When he found out she and Shawn slept together, he got mad at both of them for fooling around with the other!
On Saved by the Bell, in on episode, Slater gets mad at Zack for dating his younger sister, JB.
On Scorpion, Walter is not pleased that Sylvester has begun dating Megan. It turns out this is for Sylvester's benefit, as Walter is very aware that Megan may die of multiple sclerosis and he knows he's not exactly equipped to handle an emotional trauma like that. This becomes a source of friction in Season 2 when Walter and Sylvester disagree on Megan's medical care, to the point where Walter gets a court order and Sylvester marries Megan to go over his head. The expected friction between the two of them is cut short when Megan dies; afterwards, Walter comes to view Sylvester as a brother.
Sex and the City had a gender swapped "My Brother Is Off Limits"-plot regarding Samantha and Charlotte's brother. Samantha of course blatantly disregarded this. What does that tell us?
Stargate-verse:
- Mentioned in Stargate SG-1. To verify Daniel's identity after a body-swap, Jack asked him what color Daniel's sister's dress was when he (Jack) took her out on a date the week before. Daniel replies that he doesn't actually have a sister, but wouldn't let Jack date her if he did.
- Stargate Atlantis: Rodney McKay does have a sister, Jeannie, and expresses similar opinions toward his military best friend, John Sheppard. Her being played by David Hewlett's actual sister helps.
  McKay: She's married and she's my sister.
  Sheppard: I was just saying hi.
  McKay: I know what you were doing— I've seen that look before, Kirk.
Star Trek: Ferengi Rule of Acquisition 112: Never have sex with the boss's sister.
The Summer I Turned Pretty:
- Belly is displeased after learning her best friend Taylor has been hooking up with her brother, though it's partly because he's dating another girl at the same time.
- Steven, conversely, tells Jeremiah Belly is off limits to Conrad (by extension, him as well). He accepts it reluctantly once he's discovered their relationship however.
In Teen Wolf, Derek makes this clear to Isaac regarding Cora without even saying words...
Isaac: So, uh... your sister...
Derek [death glare]
Isaac: Sorry, it's bad timing. [Derek looks away] I'll ask later, it's fine... [another Death Glare] Or never. Yeah, yeah, I'm good with never. [Derek nods in approval]
In That '70s Show, Hyde is upset when Kelso sleeps with his half-sister. Eric, on the other hand, doesn't really care when Kelso and Fez sleep with his sister, since she's the town slut.
Trinkets: Moe is very unhappy when she finds out Tabitha and her brother Ben are getting involved and says it's really not okay (possibly because Tab's like her sister). She comes around eventually.
In The Worst Year of My Life, Again, Simon is very interested in Alex's older sister Sam. Alex tries to tell Simon not to try it on with Sam (she dislikes him, and calls him "creepy" at one point) but Simon doesn't listen.
Young Sheldon: Humorously in "A Boyfriend's Ex-Wife and a Good Luck Head Rub". Georgie spends the episode trying to convince Missy to approach the boy she likes. When she finally does so, and the boy touches her shoulder, Georgie yells at him and tells him not to touch her.

Music

As the third page quote indicates, Stephen Lynch's "Best Friend's Song" subverts this. Mark Teich is fine with Lynch sleeping with his sister...he just has to pay first.
- Note that She's in junior high It's that kind of song...
In the second music video for The Black Eyed Peas song "Bebot" Apl.de.Ap does this for his sister Jasmine.
Elton John's "Your Sister Can't Twist (But She Can Rock 'n' Roll)," in which he brags about having had sex with a friend's sixteen-year-old sister.
Die Antwoord's video for "Baby's On Fire," where the duo plays a pair of siblings, exposes the Double Standard of this trope. Ninja sleeps with every girl in town, but when Yo-Landi comes of age and tries having a sex life of her own, he flips out and chases away every boyfriend she has with increasingly-outlandish weapons. The video ends with her grabbing a brick and knocking him senseless as she runs off with her new love.
Victoria Justice's music video Best Friend's Brother, about her huge crush on her best friend's older brother. She defies the genderflipped trope, hoping to keep her crush secret from her best friend. Her defiance is only expressed in a few lines.

Mythology

Classical Mythology: Artemis usually Does Not Like Men. But she made an exception for the great hunter Orion. Her brother Apollo was not happy about this. Concerned that his sister might give up her chastity to Orion, Apollo did what any reasonable loving brother would do. He sent a giant scorpion to chase Orion and tricked his sister into shooting him. Understandably upset, Artemis placed Orion in the sky as a constellation to honor him. In some versions of the story, Apollo is deeply sorry and helps Artemis hang Orion in the sky. In other, more mean-spirited versions, it just made Apollo hate Orion even more, causing him to place the scorpion in the sky as another constellation so that Orion would be chased forever.

Newspaper Comics

In one week of FoxTrot strips, Peter is not happy when Paige says that Peter's best friend Steve is cute.
- In another, he gets upset when she attracts the attention of a lecher named Chris Morrissey. Paige assumes he's just being a pain in the butt, but when Morrissey tries to take her up to Make-Out Point she realizes Peter was right and has to threaten the jerk with mace before he agrees to take her home.
In the early days of Sally's crush on Linus, Charlie Brown would instantly get on Linus's case if Sally so much as looked at him. (Not only was Linus desperate to shake off Sally even then, but at that point in the strip's history, he was five years old, and she was only a baby.)

Tabletop Games

Champions: Try to date Green Dragon's little sister, and you've got a brand new Hunted named Green Dragon. Though this trope tends to get lost in his general control freak tendencies where his sister is concerned — he genuinely loves her, but can only express it by trying to "protect" her from all dangers (such as being able to hold down her own job).

Theatre

In On a Clear Day You Can See Forever, Edward invokes this with the song "Don't Tamper With My Sister," though the complaint here is just that it's in public (and Edward is only pretending to be her brother anyway).
In Chaucer's Troilus and Cressida, Pandarus, trying to find out who Troilus is in love with, promises to help him win her even if she were his sister.

Video Games

Art of Fighting: Ryo enforces this where his kid sister, Yuri, is concerned. Both he and his father keep a close watch on Robert and won't let him near her, even though Yuri's an adult and mutually attracted to Robert. It was eventually subverted in Ryo and Robert's endings in Art of Fighting 3 but, in The King of Fighters series, Robert's been pushed back to square one.
A rare example that is both gender-inverted and age-inverted in Black Closet: Thaïs will be very unhappy if you romance her sister Althea. She resigns the Student Council in protest (though you can convince her to come back part-time with the right dialogue options) and puts another girl up to harassing you to try and ruin your relationship.
BlazBlue:
- The game has a female example in the form of Nine. Nine is very protective of her little sister Celica and if she so much as thinks you're trying to hurt her or fool around with her (and yes, being a male in Celica's presence is all it takes to make her think that), you better hope Celica or Trinity can call her off. This gets more serious later when it's revealed that a way to stop the Black Beast would be to use Kushinada's Lynchpin. But as doing so would mean sacrificing Celica, she decided to create weapons called Nox Nyctores from countless souls. She does feel bad about the sacrificed souls, though.
- In the finale of the series, Central Fiction, Ragna starts behaving like this towards Noel as she starts the long and arduous process of reincarnating into his little sister "Saya" again. Anytime Ragna hears or sees his sister in pain, physically or emotionally, or even being flirted on (looking at you Kagura), Ragna will do his best to outright END you.
The Coffin Of Andy And Leyley has an extreme Type 4 example that appears to go both ways, as the titular siblings share an unhealthy codependent relationship. Ashley acts like a yandere to her older brother Andrew, frequently harassing his girlfriends until they break up with him and committed Accidental Murder of one when they were younger with no remorse while accusing him of trying to abandon her if he so much as looks at another woman. Andrew isn't nearly as bad as her, though he's very displeased when she jokes about sleeping with the neighbor.
In Collar × Malice, Kazuki may be the younger brother but he is usually wary of any of the love interests that Ichika ends up bringing home. Especially in Sasazuka's route where Kazuki is downright hostile to Sasazuka who enjoys mocking Kazuki by frequently calling him "little brother" and even asking if he has a sister complex. Ichika muses to herself most likely the reason Kazuki is the most antagonistic towards Sasazuka is because they have similar stubborn, aloof personalities.
Disgaea 5 gives a very extreme Type 4 in the form of Void (Dark). The source of Killia's Mangst is Void's murder of Liezerota, who interposed herself between them when Void was trying to kill Killia. Void never liked Killia, a stranger plucked off the street by his father Goldion and selected to become the inheritor of the Ultimate Demon Technique, but he became increasingly spiteful when Killia started to bond with Lieze, and Void's siscon issues found new and inventive ways to get worse from there. To put everything in retrospective, he founded the largest army in all the Netherworlds, conquered 70% of said Netherworlds, leeched Mana from a number of said Netherworlds, brainwashed his own father into becoming one Demon General, and adopted and raised another Demon General because of her power to raise the dead all just so he can resurrect Liezerota. And it still gets worse: when Void's power-infused malice is ejected from him courtesy of Killia's Macrocosm, the very first being it possesses is the just-resurrected Liezerota.
Double Homework:
- Even if the protagonist makes a deal that he and Dennis will only hit on certain girls in their summer school class, he is insistent that Johanna is not included (and neither is Tamara, who is working in the registrar’s office).
- Later exploited by Tamara when she starts dating Dennis to get back at the protagonist. He scares Dennis away from her... just like she hoped he would.
Dragon Quest VII: Zev is this to his older sister Nava during the questline at Alltrades Abbey, not approving of Eustace trying to romance her. Marible suspects it to be Type 4. By the end of the arc, though, Zev eventually relents and decides to depart on a journey so he won't interfere with Nava anymore, but not before making Eustace promise to take care of his sister.
Billy Kane from Fatal Fury and The King of Fighters is very, very protective of his little sister Lilly and does not approve of Joe Higashi dating her one bit. By the time of KOF XIV, it's gotten to the point where Joe can get Billy wanting to kill him just by mentioning her name in front of him.
Joe: You must be sweet Lilly's older brother!
Billy: I'M GONNA KILL YOU!
Fate/Grand Order: The Dioscuri Twins, Castor and Pollux, are summoned as Servants, but Pollux is given the infamous Gender Flip treatment into a girl. Castor gets angry when Pollux and their Master Ritsuka Fujimaru start getting friendly, and when Pollux compliments Heracles' abilities as a warrior, Castor asks her if they slept together. Pollux is embarrassed by her brother's overprotectiveness. Some of Castor's words imply this is because he wants Pollux for himself.
Final Fantasy XIII has a bad case of this. Snow Villiers: Boisterous Bruiser. Serah Farron: Lightning's sister. Lightning: Guardian Corps. Snow: Likes Serah. Serah: Likes Snow. Lightning: Does not like Snow. Serah: Branded by a Pulse fal'Cie. Snow: Proposes to Serah. Serah: Tells Lightning both of these at once. Lightning: Adamantly refuses to believe either. Serah: Damsel in Distress. Lightning and Snow (and Hope and Sazh): Branded by the same Pulse fal'Cie during an attempt to rescue her. Lightning: Rage Against the Heavens. Snow: Always Save the Girl. Final Fantasy XIII: Three Lines, Some Waiting. Lightning: Gets over it by the halfway point. Serah and Snow: Happily Engaged.
Final Fantasy XV: Iris Amicitia, younger sister of Gladiolus, appears to be a case of this. When she's showing Noctis around Lestallum, she remarks that it "almost feels like a date"; Noctis can remark that "if it were, [Gladio] would kick [his] ass", implying that this trope is normally the case. It probably is for every other boy who tries to date her given her expressed frustration. However this is subverted later by Gladiolus who requests Noctis go with him to pick some flowers for Iris but then asks Noctis to give them to her knowing about Iris' crush on Noctis and apparently approving. If Noctis weren't already engaged Gladiolus might actually be a shipper on deck.
Fire Emblem: Three Houses
- A particularly fierce example occurred in the backstory. A nasty noble married Mercedes' mother, because she had a Crest and having an heir born with a Crest is considered important for the status of the house. Once he did get a son with a Crest, he had no more use for the mother and she took Mercedes and fled the house. Years later, the noble finds out where they've run to and plans to make the much younger and still fertile Mercedes marry him for some more heirs. His mistake was announcing this plan within earshot of his son, a.k.a. Mercedes' half-brother. Who promptly slaughtered the entire household and became the Death Knight.
- A less extreme example occurs in Balthus and Hilda's paired ending, in which Balthus has to defeat Hilda's older brother Holst in order to obtain Holst's blessing to let Hilda travel the world with him. Keep in mind that Balthus and Holst are best friends.
- Seteth is rather hostile to those who show interest in his younger sister Flayn, even friendship. He's actually her father, making him a Boyfriend-Blocking Dad instead.
Sweet is this to Kendel in Grand Theft Auto: San Andreas. He does eventually lighten up on Cesar, but that could change if he ever pulls a Face–Heel Turn. In contrast, Carl has no problem with it and befriends Cesar quite quickly and is supportive when Cesar talks about proposing to Kendel.
In Grand Theft Auto IV Packie has this to say about his sister, Kate:
Packie: What a girl! I think she likes you. Word to the wise, though - she don't put out. Which is convenient, 'cause if she did, I'd have to kill you.
In League of Legends, the Brother–Sister Team of Garen and Lux Crownguard both end up on sides of this:
- Ezreal has a hopeless crush on Lux. While not developed in the original game, it gets a nod in the spin-off CCG Legends of Runeterra where, unsurprisingly, Garen doesn't approve:
  Ezreal: I didn't think her big brother would be so... big.
  Garen: Don't even think about it!
- Garen, meanwhile, has a not-very-well-hidden secret relationship with Katarina. Lux is somewhat more amused by it, but she's similarly protective of her big brother:
  Lux: Kat and Garen sittin’ in a tree! And you better just be holding hands, or I’m telling!
In The Legend of Heroes: Trails of Cold Steel, Rean Schwarzer has a bad case of this towards his adoptive sister, Elise, never realizing that she actually harbors romantic feelings towards him, nor taking the hints from others regarding this.
Implied in Nine Hours, Nine Persons, Nine Doors. Santa and Akane are actually siblings who pretend that they don't know each other for most of the game and Santa is literally the only person in the group who shows any kind of disapproval towards Akane and Junpei's relationship, although it's not clear that it's this trope, him not being fond of romance in general.
Odin Sphere: Ingway expresses frequent disapproval of his twin sister, Velvet, having a romantic relationship with Cornelius, the prince of Titania. Both out of spite and wanting to further his own personal goals, he ambushes Cornelius and curses him into the form of a Pooka in an attempt to keep them apart.
In Persona 3, this is played with during the ending in March. If Kenji's link is maxed out in FES, he'll mention his sister will be attending Gekkoukan during the next year, and initially has no issue with the idea of the protagonist dating her (in fact, he says it'll keep her too busy to bother him). As soon as he realizes he'd potentially be the MC's brother-in-law, however, he takes the offer back.
A rather odd inversion happens in Phoenix Wright: Ace Attorney. Larry openly hits on Maya until she explains that she's "the sister". She means that she was Mia's sister, but Larry thinks she means that she's Phoenix's sister, at which point he stops flirting. Apparently while Larry will hit on just about every cute girl he comes across, he draws the line at his best friend's sister.
Ratchet & Clank: Up Your Arsenal did this, rather disturbingly.
Qwark: I thought we agreed to put that jungle business behind us.
Skrunch: SQUARK!
Qwark: It was mating season! How could I have known she was your sister?!
Notices Ratchet and Clank (Klunk) are watching
Qwark: How long have you two been standing there?
Clank/Klunk: Too long.
In the Smuggler storyline for Star Wars: The Old Republic, one of Corso Riggs' quests involves having to contact his cousin. Should the Male Smuggler flirt with her, it will result in a massive affection drop.
Tales from the Borderlands: Played with. Rhys and Sasha have some chemistry, and Sasha's sister Fiona disapproves. However, since the player controls both Rhys and Fiona at different points, this can change. Rhys can deny the chemistry, pursue Sasha more aggressively, or just allow things to progress as they may. Fiona, on the other hand, can either give her blessing or tell Rhys to stay away from her sister.
A mild example from Tales of the Abyss comes in the side-quest where you have to find Emperor Peony's pet rappigs. He's named all of them after people he knows, and the one named for Jade's sister Nephry has the nicest collar. Given that Peony and Nephry have a (long-past) history anyway, Jade is noticeably displeased by this discovery.
Trillion: God of Destruction: Zeabolous is extremely protective of his sister Elma. He shows considerable animosity towards Uriel whenever he tries to make a move on her.
Averted in Xenoblade Chronicles 1. Dunban quite openly ships his little sister Fiora with Shulk.
This will basically be Hanako Yamada's role in Yandere Simulator: in the week where she is the rival, she will be extra clingy toward her brother, Senpai, and her 'confession' will be her asking him not to get a girlfriend. Yandere-chan will be wanting to stop that request by any means necessary. (It is noted that there's one exception to her brother being off limits: if he ends up in a Childhood Friend Romance with Osana Najimi, she'll be okay with that.

Web Animation

Homestar Runner:
- When Coach Z implies that he fancies Bubs' sister in "Most in the Graveyard", Bubs reacts by punching Coach Z in the face.
  Coach Z: So your sister and I can— [punch]
- Volume 4 of the Strong Bad Email DVD complation includes a bonus email, "Family Resemblance", where Strong Bad imagines what Pom Pom's hypothetical family looks like. It includes an Easter egg where Pom Pom catches Strong Bad flirting with his hypothetical little sister Stella Do (a walking set of headphones with a disembodied face).
  Strong Bad: So, Stella Do, wanna come over to my place tonight? I could maybe plug you into my record player...
  Pom Pom: (bubbles angrily)
  Strong Bad: Oh, Pom Pom! Uh, look man, it's strictly hypothetical between us, I swear. I never touched her floaty face!
The "Bro, are you flirting with my sister?" meme.

Webcomics

In The Dreadful, nobody fucks with Kit's sister. Ever! Unless they want bullets as a last meal.
In this strip of Loserz, this trope is invoked when talking about Adam's little sister.
Madame Outlaw: In a flashback to Estelle's first ball, her brother Isaac is annoyed when his friend Thaddeus asks her for a dance.
Ryujin of Mob Ties informs Sidney Burns that if he marries Mika, Ryujin's sister, he will kill Sid, in the very first issue. This puts Sid in a bind, as Papa-san, Mika's father, will kill Sid if he doesn't marry Mika.
Played with in Panthera, as Jason is clueless about Fletcher taking a liking to Taylor until Kira points it out to him — when he promptly goes into stunned denial of the possibility precisely because Fletcher is his best friend.
- Now that Kira told him, though, he's a completely straight example of this, with some Knight Templar Big Brother thrown in for good measure.
Gender switched in Questionable Content where Dora gets very upset about the idea of her friends dating her brother.
- Two reasons behind this. One being Sven is very anti-commitment and is likely to cheat (Which he does). Second, she reveals to Marten that a lot of girls in high school only pretended to be her friend so they could seduce Sven and she still resents it.
This scene from Wilde Life:
Oscar: River is my sister.
Cliff: She hot?
Oscar: Is she—she's married and you're twelve.
Cliff: Sixteen!
Oscar: You know what? I'm not comfortable debating my sister's ostensible hotness with a teenaged werewolf. (throws up hands) And I didn't even know that about myself until today. What a magical journey of self-discovery this has all been.
The ghost of Gray Fox in The Last Days of Foxhound is usually placid in the afterlife, reflecting upon his life of slaughter and deciding that he doesn't want to fight anymore. That is, until Liquid Snake lets slip that he's dating his sister. Liquid and Naomi went on one date and don't even like each other; Liquid panicked after Sniper Wolf asked him out (because she kills almost all her crushes) and and asked out the first woman he saw, and Naomi agreed because she wanted to kill Liquid.
In Lackadaisy, Victor clearly sees Ivy as a surrogate daughter or perhaps niece. He apparently beat up her previous three boyfriends, at least one to the point of putting him in a wheelchair. Ivy is not happy. So far, Freckle has managed to survive in one piece, but Victor has also been stuck recovering from a sucking chest wound for the duration of their relationship...

Web Video

Three examples in Critical Role — two of them have Vex and Vax pulling this with the other's developing lover (though Vax had a legitimate excuse after Percy's failed Perception resulted in Vex's death), but they eventually become shippers on deck. The third sticks — Scanlan backs off of Percy's sister Cassandra when he does not approve of his attempt to impress her.
During a session of Oxventure's DnD campaign, Merilwen immediately tells Prudence upon meeting her father that she is not allowed to crush on him. She is then told by Dob that all of the group, including the men, are already crushing on her dad.

Western Animation

Arcane has a much darker version of this trope; Jinx thinks that her older sister Vi is dating Caitlyn and gets very angry and jealous as a result, since this feeds into her fear of Vi abandoning her and finding someone else as a "replacement". It gets to the point that she kidnaps Caitlyn and pushes Vi to kill Caitlyn so they can "go back" to being sisters.
All Grown Up!:
- Chuckie takes offense when he thinks Tommy has romantic feelings for his sister, Kimi. It's later revealed that it was the other way around, and only for a few minutes, though at the very end it's implied that Tommy and Kimi might actually have feelings for each other after all.
- He was also very overprotective of Kimi when bad boy Z started to "take interest" in his little sister. She certainly didn't help her case when she dyed her hair and started wearing different clothes.
  - Funnily enough, Chuckie dealt with being at the other end of this trope years earlier. In the Rugrats episode "He Saw, She Saw", he gets a crush on a little girl named Emma, who has a big, chubby brother, Big Binky, who is also very protective. Thankfully, Big Binky befriended Chuckie when the latter saved Emma from an infestation of ants.
- Inverted in another episode when Phil actually begs Tommy to ask Lil to dance so she'll leave him alone.
- Also played straight when Phil chases off a guy talking to Lil when they begin to realize their gender differences.
The main ¡Three Amigos! of Avatar: The Last Airbender is of the brother and sister + a friend variety. Friend Aang ends up marrying the sister, Katara, but this is only ever really brought up once. In the episode “The Fortune Teller”, Aang eavesdrops in on Katara getting told she’s going to marry a powerful bender. The episode is pretty early in the series, but he’s been harboring a crush since he met her. He decides to make a move and go to the brother, Sokka, for advice because Sokka is a few years older than him. A classic One Dialogue, Two Conversations ensues in which Sokka thinks that Aang is talking about one of the girls that lives in the town and Aang thinks Sokka is giving him blessing about liking Katara. Aang even asks him, “You’re okay with it?”.
The main character from Danny Phantom is absolutely livid when his older sister is dating a biker named Johnny and spends the episode splitting them apart. He's a ghost who plans to use her body to host his equally ghostly girlfriend, but still.
Inverted in Dexter's Laboratory when Mandark develops a crush on DeeDee. Dexter encourages this, because it means his sister will be wreaking destructive havoc on Mandark's lab instead of Dexter's.
Family Guy: Quagmire goes on an absolute rage when he learns Brian had sex with his transgender father (which Brian had no knowledge of).
Quagmire: (to Brian) If I ever see you anywhere near my house again, I'll blow your head off! (delivers a few more punches before leaving) Now lay there and die, you piece of crap!
Jimmy Fitzsimmons of F is for Family forbids any boy from dating his sister, Bridget. In a spin on the usual formula though, he's protecting them from her, as Bridget is a complete and utter lunatic.
In the Catillac Cats segment of Heathcliff & the Catillac Cats titled "Games of Love", Mongo meets a pretty Southern girl named Dixie training for track & field. The two are smitten with each other –- much to the annoyance of Dixie's muscular older brother, Jake. Jake pays a visit to the junkyard later to warn Mongo to keep away from his sister. This stems more from not wanting his sister distracted from her athletic training than not liking Mongo.
In He-Man and the Masters of the Universe (2021), Adam and Krass are Like Brother and Sister. When Teela arrives at the Tiger Tribe and Adam shows interest in the stranger, Krass is bitter and sulking about it. While Cringer initially teases Krass that it may be romantic jealousy, it's clear that Krass is worried that Adam will one day leave the Tiger Tribe (and her) to seek a different life.
In The Legend of Vox Machina, Percy shuts down Scanlan's attempt to hit on his sister, Cassandra, with a curt "No."
In Next Avengers: Heroes of Tomorrow, four of the protagonists were raised together by Tony Stark, but Francis^noteson of Hawkeye and Mockingbird was not. When he explains his decision to join their team:
Francis: Besides, anything to impress a girl, right?
James^noteson of Captain America and Black Widow: Girl?... Wait, you mean Toruun^notedaughter of Thor? Dude, she's like my sister!
Francis: So...she's available then?
James: NO!
In The Penguins of Madagascar Hans the Puffin tries to do this to anger Skipper in "Huffin And Puffin". It kind of fails...
Hans: I kissed your sister! On the lips!
Skipper: I don't have a sister, and even if I did she wouldn't have lips!
Hans: Really? Then who did I kiss?
In Phineas and Ferb, Suzy Johnson has the Type 4 with her older brother Jeremy, and makes Candace (Jeremy's crush/eventual girlfriend) miserable as a result. There are a few times Suzy is cordial to Candace, but it's usually only when Jeremy is nowhere near her at the moment.
Les to Noam in Rated "A" for Awesome.
Les: Dude! That's my sister!
The Simpsons:
- In "Colonel Homer", Krusty is shown whacking Sideshow Mel around for dating his sister. After Lurleen's song plays on the radio, Krusty relents, gives Mel some money and tells him to take her somewhere nice.
- In "She of Little Faith", Jimbo attacks his friend, Kearney, when he finds out he's dating his mom.
  Kearney: Hey, she came on to me!
- When Lisa starts dating Nelson in "Lisa's Date with Density", Bart tries to get her to break it off.
  Bart: I'll probably never say this to you again, but you can do better!
- Subverted in another episode, a boy asks Bart for permission to date Lisa. Bart looks completely relaxed but he never answered the question.
In Transformers: Prime, Miko, whose affection for Bulkhead has never really been portrayed as romantic, nonetheless gets extremely jealous in the episode "Metal Attraction" when everyone teases Bulkhead and Arcee after a powerful artifact magnetically sticks them together.

Real Life

This is the story behind the band No Doubt. Gwen Stefani's brother, Eric, started the band and she liked fellow band member, and friend of the family, Tony Kanal. He thought she was off limits, but they ended up dating for a while in secret. The relationship was less than successful, as reflected in some of the group's songs like "Don't Speak". Everything turned out okay in the end, and both are happily married to other people now. She sang about this in the song "Cool", indicating that they had closure.
Maxim Magazine codified the 24-hour rule (mentioned in the description above) as a "Man Law."
Zinedine Zidane's famous headbutting of Marco Materazzi in the 2006 FIFA World Cup Finals came after Materazzi said 'I'd rather have your whore of a sister' when Zidane offered to give Materazzi his shirt, which he'd been pulling on. To Materazzi's surprise, he made that boast without knowing he had a sister.
Among some traditional people, like the Maasai in East Africa, it is common for young men of the same age to form a group of warriors. They consider the others in the group their brothers and hence, dating a "brother's" sister is considered equivalent to incest and as unthinkable as dating your own. (Most traditional societies take social relatedness very seriously. For example, there is such a thing as milk siblings — having been breastfed by the same woman makes people siblings, which makes sense if you consider that having been raised together they will view each other as siblings, regardless of genetics.)

My Sister Is Off-Limits

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