A major Truth in Television trope, as most any guy can tell you. If you've known someone for more than 24 hours, 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.
If the couple breaks up, the friendship is over.
The boyfriend (and in some cases the brother) may have a history of being, shall we say, "less than respectful" of their "conquests" and the brother doesn't want his sister getting the same treatment.
The brother is creepily jealous of any guy who gets close to his sister.
While the title says "sister," variations of it also include mothers and other female relatives (such as cousins), and for girls this would most likely be "My Brother is Off-Limits." This doesn't include daughters, because Overprotective Dad already covers that. 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. Contrast Best Friends In Law, where the guy is all too happy for his good friend to end up with his sister.
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Anime & Manga
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.
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 coworkers gave us this wonderful quote:
In Rave Master, Haru does this trope when he learns of the relationship between his sister Cattleya and Shuda.
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.
The Yu-Gi-Oh! anime has type four, or maybe type five with Joey and his sister, Serenity. This mostly affects Joey's best friend, Tristan, who is nuts about Serenity (while Serenity herself sees Tristan 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.)
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.
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.
A reversed example in Infinite Stratos: Chifuyu Orimura, possibly the world's strongest IS pilot, is very much aware about 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.
DO NOT get any ideas of dating Noa's brother in Oniichan ControlSHE 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.)
Played straight in Ore no Imōto ga Konna ni Kawaii Wake ga Nai. 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.
The Black Butler manga has a Type-3/Type-4 variant in Elizabeth Midford's elder brother Edward: he's not that much in favor of his sister's fiance at first, but has slowly mellowed out in latter chapters...
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.
And then there's Ultimate Quicksilver, who doesn't want anyone dating his sister because... well...yeah.
In the X-Wing Series 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?"
Falling under the "extended family" (of sorts) 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.
Carferrix's attitude to his sister Chipolata in Astérix in Corsica.
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 If Thems 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.
Stallionsof 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!
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.
The brothers in Fools Rush In seem to be like this, at least that's what Alex fears.
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.
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?
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?
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.
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 major factor in Living Will, with Belcher upset about Will and Krista's relationship.
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.
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...
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 restored timeline Marco only ever flirts with her. In either case, Jake doesn't seem to care.
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.
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, 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.
This attitude is the death of Margarita's brother in many versions of the Faust legend.
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 Peter Tinniswood's novelExcept 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, Cater 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...
Live Action TV
Game of Thrones: A non-verbal version is featured in "Valar Morghulis." After Ser Loras Tyrell unhappily offers his sister as a bride to King Joffrey, he then stands protectively behind Margaery Tyrell and stares defiantly in Joffrey's direction. The boy-king is only paying attention to Margaery at this point, so he doesn't get the message, but Loras is warning Joffrey that he had better not harm his sister. Given it's Joffrey, this is understandable.
The Fresh Prince of Bel-Air. Carlton with Ashley. With Will its more of a case of 'My Cousin is Off Limits'. Strangely neither of them care about who their older sister/cousin Hilary dates or sleeps with. In fact Carlton tried to get into university by advising the university rep how to get Hilary into bed and advised her to get out of problems with the IRS by wearing sexually provocative clothing.
It's not that strange - Ashley's a teenager, while Hilary's the oldest. There's a reason it's called a Big Brother Instinct.
Additionally Hilary's implied to sleep around quite a bit, so Will and Carlton probably see no point in trying to protect her.
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 wanted to take Carol to a dance. Mike was upset at first but later accepted 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.
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.
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?
In How I Met Your Mother, The Casanova Barney makes it clear note "It's like listening to Weirld Al Yankovic, if Weird Al Yankovic only sang Christmas carols about banging my sister." 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.
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.
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.
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.
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. 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.
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.
Rodney McKaydoes 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.
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.
In NCIS, Di Nozzo 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...
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...
In Teen Wolf, Derek makes this clear to Isaac regarding Cora without even saying words...
Subverted in 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!"
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 who 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.
Actually, in the Leonard/Priya instance, it was Priya who was hitting on Leonard, and Raj, oblivious of this fact, was quite upset with Leonard.
Tommy gets a nonverbal version, over Oliver's teenage sister Thea.
Tommy: Have you noticed how hot your little 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.
Now pretty much becoming a Running Gag. Once more with feeling when Tommy visits Laurel and finds her father there. They're dating at the time, mind you.
Tommy: Merry Christmas. How are you? Det. Lance(in a low voice): Proficient with firearms.
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.
Subverted on Chicago PD, 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.
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.
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.
Star Trek: Ferengi Rule of Acquisition 112: Never have sex with the bosses sister.
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.
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. This just made Apollo hate Orion even more. He placed the scorpion in the sky as another constellation so that Orion would be chased forever.
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 Chris Morrissey. Considering that Chris is quite lecherous, Peter is completely right this time.
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? Klunk: Too long.
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.
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."
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.
Blazblue 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.
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.
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.
In this strip of Loserz, this trope is invoked when talking about Adam's little sister.
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.
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 may actually have a thing for each other after all).
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 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.
In the Catillac Cats segment of Heathcliff and 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.
On The Simpsons episode "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!
This is actually 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 did end 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, though, and she is happily married now, and he is also in a relationship. 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 World Cup Finals came after Materazzi said 'I'd rather have your sister' when Zidane offered to give Materazzi his shirt, which he'd been pulling on. Or something like that.
Among some traditional people, like the Maasai in East Africa, it is common that young men of the same age form a group of warriors. They consider the others in the group their brothers and hence, dating a "brother's" sister is as unthinkable as dating your own.