Romantic or sexual attraction between siblings. Most of the time it may be merely implied, but sometimes it's laid out right in the open for the viewer to see. Its presence in a story usually adds a great deal of emotional intensity.
Frequently actual incest is avoided through the device of siblings who aren't really they're fostered, or step-sibs, or adopted. Thus, while in arbitrary terms of relationship they may be brother or sister, in "true" terms of blood they are not, and may pursue their chosen target with relative impunity. Often it's just an extreme version of the Childhood Friend Romance setup; male and female characters who normally couldn't cohabitate or possibly even interact normally with each other are 'forced' to but meet with an arbitrary contrivance preventing them from developing past it. The only difference is that the audience is more likely to accept the latter contrivance as believable.
Many sociologists and evolutionary psychologists believe that the instinctual aversion to incest has developed to avoid the pairing of rare, potentially lethal, recessive genes. The incest taboo is implemented through the Westermarck effect, in that growing up in proximity at a young age (through approximately six years of age) renders it sufficiently likely that the persons are siblings, to trigger the instinctive aversion. Many close tribal and small village communities marry outside of the tribe or village, and many childhood friends end up unlucky, because of this. This would also explain why brother-sister incest was possible among Egyptian royals (where the girls were raised separately from the boys and didn't even meet until after puberty), while some European royal families such as the Valois faced extinction because the king and queen couldn't bear to touch each other not because they were that closely related, but because they'd been brought up together since early childhood and thought of each other as siblings.
Although the Japanese have just as much of an incest taboo as any other culture, there does not seem to be any automatic assumption of tragedy surrounding incestuous relationships in anime, as there would be in most Western productions. It may be shown as sad, improper, and/or perhaps even reprehensible, but participants are no more likely to come out to a bad end than any other characters in the story. A supposed Japanese legend which states that star-crossed lovers are reborn as siblings probably has something to do with it as well.
It is also very common in Japanese works, at least, to see a girl address a boy she's interested in as onii-sama, the ultra-respectful form of... 'big brother'. For example, in Mai-HiME, Munakata Shiho addresses Tate Yuuichi in exactly this way, and harbors an obsessive desire for him that leads to some fairly serious jealousy of her rival (Tokiha Mai). This is also a common trope of bishoujo games. Of course, this is more a Japanese language trope; onii-sama or onee-sama ("big sister") is a fairly standard thing to call someone older than you that you respect.
Contrast with Parental Incest, where the parent/child power dynamics can make it even squickier. Not to be confused with Double In-Law Marriage, where both of the couples are Not Blood Siblings but siblings-in-law. May show up in works with a Little Sister Heroine.
NOTE: This trope is for actual cases of incest in-story. Incestuous subtext goes in Incest Subtext.
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- Western Animation
- Sometimes used as a substitute for Parental Incest in fairy tales, such as Penta of the Chopped-Off Hands.
- In Princess Belle-Etoile, Belle-Etoile struggles with her feelings toward one of her brothers, Cheri, and his toward hers. Until she overhears a conversation between her parents, and finds they were actually all foundlings, and various things about them made even the parents think that he might not be her brother. In reality, he's her double first cousin.
- The Lion King may or may not contain a half-sibling example with Simba and Nala. While any shared parentage is never made explicit, the movie never addresses the fact that Mufasa, as king of the pride, would normally be the one to father any cubs, and that Mufasa and Scar (who is stated never to have fathered any cubs) are the only male lions anywhere to be seen. The spinoff cartoon The Lion Guard showed that Nala's father is an unrelated male but it contains inconsistencies with the films and Disney doesn't acknowledge spinoffs or most sequels, so it's still rather ambiguous who Nala's father is.
- Hercules doesn't mention it, but technically, Hera and Zeus are siblings (as are the rest of the gods).
- Discussed in Fat, French and Fabulous: This may have been the case between the Papin sisters, who suffered from shared psychosis.
- Vince Russo planned on doing this as a plot element to the Stacy Keibler pregnancy angle. She was seeing David Flair in the storyline, and was his fiance, so Russo planned to reveal that Stacy was Ric Flair's illegitimate daughter. Fortunately, WCW went out of business before this angle could come to fruition.
- Mark Henry claimed during a sex therapy session that he lost his virginity to his sister when he was 8 years old and that he had just had sex with her 2 days ago.
- Vince McMahon of World Wrestling Entertainment has been trying to do an incest storyline going all the way back to 1999, when Ken Shamrock was supposed to be in love with his Kayfabe sister (Shamrock refused). The Mark Henry case apparently didn't satisfy him so Vince tried to have an incest storyline between himself and his real-life daughter, Stephanie (she refused); he countered by replacing himself with his son Shane (both of them refused). He finally got his incest storyline between Paul Burchill and his Kayfabe sister Katie Lea, although it never got past Incest Subtext. Note that his inability to get an incest story actually going didn't prevent him from alluding to such things.
- Vince's attempts to do an incest storyline prior to the WWE going PG is most likely due to issues involving his screwed-up childhood. He unfortunately vents all these issues through storyline, instead of venting it to a psychiatrist like anybody else would. As Vince is someone whose blood would test positive for Kayfabe if such a thing was possible, it may be impossible to find out what really makes him tick.
- This has cropped up in Survival of the Fittest a couple of times (although in one case this was Twincest). Matthias Kovalenko of V2 was scarily obsessed with his sister Jodeen. (fortunately, she never made it onto the island). Nothing explicit ever occurs, but from Matthias' thoughts, most get the impression it isn't just brotherly concern.
- Simon Bar Sinister and Penny Dreadful were a brother-sister team of supervillains in the Global Guardians PBEM Universe. In addition to being murderous psychopaths, they were utterly and completely in love with one another, to the point of getting physical while they were in the middle of a crime.
- In The Gamer's Alliance, Desdemona and Mordecai are siblings and very much in love although they have to hide their relationship from everyone else. Desdemona eventually gets pregnant with Mordecai's child and intends to give birth to the child, using it as a means to install herself on the Remonian throne by claiming that the child belongs to King Marcus.
- Fortunately averted in Broken Saints: Shandala and Gabriel look like they might be bonding romantically prior to Tui Jr's death, but it is cut mercifully short. This is especially creepy when you realize Gabriel knows Shandala is his sister.
- Lilium -Sims 2 is a Sims 2 machinima that revolves around a rich, isolated family circa 1900. Isis is attracted to her (possibly twin) brother Horitio. Their father doesn't like this so he has them meet the set of siblings he had paired them up with as children. Horitio is both in love with Isis and Estella, while Isis has no feelings for Baltazar and only wants Horitio. The siblings end up marrying their arranged spouses, but only one of the couples is happy.
- The story "Becka the Beast" (NSFW) is about Gia Cameron, a karate black-belt who falls in love with gay school bully Becka Jackson. It's quickly revealed that Becka is the victim of Parental Incest, coupled with a relationship with a Depraved Bisexual that has left her an empty shell. Gia's father adopts Becka in the end.
- In the Chakona Space setting, there are a number of examples of this (as well as some Parental Incest) between members of various genetically-engineered species such as chakats and foxtaurs. Hermaphrodite chakat sisters Forestwalker and Goldfur enjoy recreational sex, and foxtaur brother and sister Garrek and Malena are also mated (this is hand waved with the explanation that they have been genetically engineered to eliminate the sorts of diseases and conditions that inbreeding can produce in humans, and as created species do not share the exact same moral squick factors as their creators). Granted, Chakats and most other species still have taboos against inbreeding so Goldfur and Forestwalker are careful not to have sex when either one is in heat. Foxtaurs allow one generation of inbreeding because most of them can't leave their isolated Amish-style villages thanks to Territorial Attachment Syndrome, and a shortage of males.
- In Tea, Biscuits and Incest Mikayla eventually learns that her boyfriend Jayden is also her half-brother.
- Present in a benign form in regards to Ben in An Ordinary Sex Life. Adrienne on the other hand...
- Appears in the Haskhian Inscriptions, a book in The Wanderer's Library, as it's based on ancient mythologies.
- In an online Q-and-A session, Invoked in the Wizards of Waverly Place creator's non-denial denial response to "a Jalex question":
Oh my gosh, we got a Jalex question.
(Reading) "Are Justin and Alex going to end up together?"
(Laugh) They are brother and sister, people.
- In SCP-404 of the SCP Foundation, a small experiment log tells the story of a Class-D having a questionable relationship with his sister when they were pre-teens and killing her boyfriend out of jealousy.
- In supernatural web soap California Heaven , an episode features brother and sister, Dominic and KC, in a very seductive dance nearly kissing. Thought, it looks like KC was possessed. However, in a most recent episode, Dominic stares at KC as she is sleeping and she seems to have no problem with it.
- Implied in Spooning With Spoony III, when Spoony fucks both The Nostalgia Critic and The Other Guy (along with plenty of other people) and tells the latter he makes noises just like his brother.
- This was to have been a plot point in The Nostalgia Chick's video "Thanks for the Feedback" detailing a date between Chick and the Critic. In the original draft, they would have slept together and then been informed that they were actually brother and sister. This idea was scrapped prior to filming.
- In Comix from the Underground, the Comix Scrutinizer suggests that the furry webcomic characters Kit n Kay Boodle are participating in this since they're not married and have the same last name.