Popular culture often expects twins to share everything, including, it seems, their beds. It's a particularly common trope in Slash Fic and Rule 34, though it does occur in mainstream fiction often enough to be a discernible trope. It's especially common as a way of making Creepy Twins just that much more disturbing.
Note that Twincest is not necessarily homosexual: that's what fraternal and Half-Identical Twins are for. However, a great deal of the appeal to viewers is the popular fantasy of sleeping with two identical copies of a single hot person at the same time, so homosexual (or bisexual if there's a lucky third party of another gender) twincest is more often used to titillate viewers. For Yuri Fans and Yaoi Fans who became fans at least partly because they see a forbidden appeal to such relationships, Twincest supercharges that part of the appeal.
A common twist, in this and in Brother-Sister Incest in general, is for a couple to fall in love, only to find out later that they were Separated at Birth. (See genetic sexual attraction.) How far their relationship goes before they discover this generally depends on whether the plotline is supposed to be humorous or tragic.
It goes without saying that most Real Life twins are as completely repulsed by the idea as any other incest trope.
An old Japanese myth says that if two star-crossed lovers commit dual suicide, they get reincarnated as twins. This may be one of the factors in the frequency of Twincest stories in anime.
Of course, sometimes even Twincestisn't quite incestuous enough for the writers.
Can overlap with a Twin Threesome Fantasy.
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Anime & Manga
This is the main "schtick" of Hikaru and Kaoru from Ouran High School Host Club: making the club's female clients swoon and sigh over "the forbidden love between brothers!!!11!!!" Whether it's a put-on or not is hard to tell, given their trickster personalities.
Also deconstructed, as the twins are revealed to be dangerously codependent. In the manga, this eventually sparks a feud between them, and they eventually learn how to be individuals.
Candy Boy is entirely about yuri twincest, in that Yukino does like to tease her sister Kanade quite a bit. There's nothing explicit going on, since Kanade decides to keep a cool outer appearance, although she is obviously enamored with her sister.
Something explicit IS going on by the end. They kiss in the last non-DVD episode.
The manga also revolves around their romantic, and sometimes sexual, feelings for each other.
Subverted in Gundam SEED: Kira and his Separated at Birth twin sister Cagalli meet in the first episode (neither knowing they're related), and when they reunite further in the series, Cagalli comforts the emotionally ragged Kira in a purely platonic way. This is ironically taken as a threat by Flay, Kira's screwed-up girlfriend (it's not a healthy relationship), despite the fact there's no romantic attraction to it. Further toyed with when Cagalli displays a Luminescent Blush at overhearing Flay admit to her ex-fiance Sai that she and Kira have slept together, and when Kira's adoptive parents Caridad and Haruma (who are aware of Cagalli's identity, as the mother rescued both twins as babies from the place where their bio parents died) show a bit of alarm when Cagalli hugs Kira goodbye.
Gundam 00 plays with this too. Whether RibbonsAlmark and his "sister" Hilling Care qualify as twins is debatable, both of them being DNA-engineered children with the same sequence, but Healing seems to show a bit more adoration than is platonic towards Ribbons.
In Monster, there is a one-sided example with Johan Liebert and his twin, Anna. Unfortunately, Johan is also reallyscary and effectively scares Anna half to death whenever she thinks about him. Although, considering how he treats other people, this may be the only way he's even capable of displaying affection whatsoever. It doesn't seem to be brotherly love, though, when he talks about her as his "other half". It comes off as a really creepy infatuation, usually.
.hack//tasogare no udewa densetsu hints strongly at incest, or at least lust, between two characters who are twins. This is basically a Flanderization of subtler jokes in the manga, where it was the sister's completely different-looking and skimpily-dressed computer avatar that set off the whole thing. The anime altered the storyline near the end where their relationship is implied to be much stronger.
Onegai Twins features a boy who is not sure which of the two girls in the story is his long-lost twin sister. They both fall for him, making one a case of twincest and the other not. Now, if only they knew which... If only the manga and anime were the same on that point.
Chobits has some extremely un-sibling-like touching between Chi and the "Dark Chi" that appears in her mind. This proves to be her sister, Freya, whose mind was downloaded into Chi.
On learning that Fai of Tsubasa Reservoir Chronicle has a deceased twin brother named Fai (hence why the Fai we know and love is named Fai), the fangirls hit the keyboards hard.
The Acid Tokyo arc's Kamui and Subaru (also known as the "Vampire Twins") actually manage even more Ho Yay than their counterparts in X1999 (where they are not related in any way). Vampirism (and new twincest possibilities) may have something to do with it.
Mercifully, the fanfic authors do not do this with Syaoran and Sakura and their manyclones, as that would likely cause the already inscrutable relationship chart (for their fic, anyway) to actively kill those that look at it.
Myself ; Yourself has twins Shuri and Shusuke with a particularly strong relationship in the face of Parental Abandonment, with a later rumor even claiming a more illicit relationship. Under threat of separation, they run away together near the end. The Distant Finale strongly suggests that the subtext has become canon.
The Kannazuki no Miko manga ends with Chikane and Himeko reincarnated as twin sisters. This may be a variation on the Japanese myth mentioned above.
Let's not get started on Rociel / Roshiel's 'ambiguous' attitude towards his twin sister Alexiel in Angel Sanctuary. But then again, Rociel isn't a paragon of sanity, and brother-sister incest is one of the main themes of the manga.
Zero and his long-lost twin Ichiru in Vampire Knight. The 'cest is entirely irrelevant to the plot. It just needed that extra element.
In an example where the incest is relevant to the plot, in SaiyukiHakkai's girlfriend Kanan was actually his Separated at Birth twin sister. It's not known when they found out, but it was Hakkai's motive for what he did after she was given up to the demons by the villagers, gangraped, forcibly impregnated, and then Driven to Suicide. And apparently even after they found out, they didn't actually break up or anything.
The Hayasaka twins from Buso Renkin, while probably not involved in actual twincest, do act a lot like a married couple, and used to recite the traditional wedding vows to each other as a game, as kids.
In Full Metal Panic! The Second Raid, the assassins Xia Yu Fan and Xia Yu Lan are twin sisters who take showers together. And hold each other at every available opportunity. Also Yu Lan sees Yu Fan descending naked from the sky toward her when she dies.
The relationship between Carossa and Melissa in GUN×SWORD has elements of this, though they are never so pronounced as when the two first appear and Carossa licks away Melissa's tears. Van, watching, cannot help but be a little disturbed. They are basically kids. What was there was creepy, particularly Carossa managing to imply "Abusive Relationship" while being all of twelve years old.
Ayashi no Ceres plays this trope quite creepily and mixes it with Rape as Drama. Also averted, in that the actual twins, Aya and Aki, care for each other deeply but don't have twincesty feelings for each other; they are, however, possessed by their ancestors, Shisou and Ceres, who were husband and wife.
Suiseiseki and Souseiseki, a.k.a. the Boatlight Twins, from Rozen Maiden get this kind of treatment a lot through fan shipping. To be fair, they are an official pairing in the series itself, so it works out pretty well overall anyway.
Though barely present in the main story of Earthian, other than a mention now and again by his assistant and close friend Raphael, Lord Michael and his twin sister Lucifel are featured prominently in an extra story, written years following the original publishing, that was included in the reprint. In the story, not only is Lucifel shown to be extra touchy-feely with Michael (not to mention possessive of him, to the point where Raphael considers her his romantic rival), it is strongly implied that not only do they eventually have sex, but Lucifel has a child by him, that child being Kagetsuya, one of the two main characters of the main story. This becomes somewhat less of a shock when you consider the mangaka is Yun Kouga (see Loveless).
Yosuga no Sora is the embodiment of this trope with the male lead Haruka having such relationship with his very twin sister Sora. Among the both of them, Sora was shown more crazy and obsessed with her brother as she constantly fantasizes having sex with him and gets jealous when Haruka interacts with other females.
D.N.Angel features some Les Yay between twins Risa and Riku, though mostly in color illustrations. There's a rather eyebrow-raisingly sensual pic of them laying side-by-side on a bed. One shall also note that they get a pinky thread of love together.
Masako and Kanba from Mawaru-Penguindrum. sort-of: Masako openly chases after Kanba, but though he cares for her, he doesn't reciprocate.
This makes up all the drama in one case in Detective Conan. Raita Banba, a high school friend of Kogoro's, had a fiancee named Hatsune Kamon. On the eve of their wedding, Hatsune was killed when the car exploded, and Raita was the main suspect due to his Crazy Jealous Guy tendencies. It was later revealed that they were Half-Identical Twins, who were separated when they were babies after a house fire, and the two didn't know that they committed twincest until Hatsune received a call that the DNA results confirms that. Already very stressed from the thought of Raita's and her origins and the fact that she was an Ill Girl, poor Hatsune soon fell into the Despair Event Horizon and as a result, she committed suicide.
A Cruel God Reigns: Eric and Valentine. Eric became dangerously dependent on his twin sister after being molested by their piano teacher as a child. Unable to separate comfort from romance, the two had sex and Valentine had a baby at 13 years old.
Marvel Comics' Fenris twins (Andrea and Andreas Von Strucker) play with this idea (and likely, each other!) fairly frequently. Shudder.
And then it got worse gets worse. Andrea has since died, and since their energy blast powers relied on them touching to function, her brother had her skin made into leather and wrapped his sword hilt with it. On top of that, he is nothing short of fanatically obsessed with finding a way to resurrect her. The whole thing is portrayed just as creepily as it sounds, with even other characters around him being squicked by it. Their Ultimate versions were pretty much the same, save for the bit in spoilers.
However, when Andrea is later cloned, her brother outright denies that their relationship was ever incestuous. He allows her to sleep in a separate bed, and claims that while he knows his teammates think that he's a freak, he has no actual desire to have sex with his sister.
Top 10's Spin-Off miniseries Smax has Smax and his sister Rexa admitting to this, although in the world they came from it wasn't the taboo issue it was in his adopted world.
They are clones rather than twins, but the girls in the XXXenophile story "Family Reunion" probably count.
Aleksandr and Aleksandra Ramanov in Nikolai Dante are like this, even taking time out of killing the Raven Corps to make out.
Fandom & Fan Works
America and Canada, the "North American Brothers" of Axis Powers Hetalia, are (assumed to be) twins and pairing with growing popularity in the fandom. It can't match the popularity of the America/England pairing yet, but it has its devotees.
It doesn't stop there, either. SouthernItaly and NorthernItaly, although not actually twins, look a lot alike, and are thus confused as twins. As a result, Itacest has a pretty loyal following.
There is an Evangelionone-shot fic where a mixup occurs and another Rei is deployed while the previous one is still alive. Cue the two meeting up at their apartment and initiating some quick lesbian action... before it turns out that Rei was fantasizing the whole thing. During class, no less. Overlaps with Screw Yourself, naturally.
Yoh and Hao in Shaman King are twins. In the anime, especially the 4Kids dub, Hao kept saying it was Yoh's destiny to unite with him. A lot of yaoi fangirls love Yoh/Hao. They are literally a single soul that got split in two, simultaneously explaining the "unite" Double Entendre and making this closer to Screw Yourself.
The Hitachiin twins in Ouran High School Host Clubplay this up for their fangirls for the purposes of their club. They also seem to be very affectionate toward each other when the fangirls aren't watching, but this, too, could also be a game they play. Nevertheless, on one occasion where Tamaki calls them early on a weekend, we see them waking up in bed... the same bed... cuddling each other. Naked.
Dipper and Mabel from Gravity Falls, due to the fact that they always hug or hold hands.
There are multiple examples of pairing up Netto and Rockman from Mega Man Battle Network/Mega Man NT Warrior. It's up to the Gentle Reader to decide which turns on or squicks him or her more — the game version, where this trope applies in full, or in the anime, where he's just a computer program.
Amiboshi and Suboshi in Fushigi Yuugi, though a lot of this is fan speculation. Especially because Suboshi feeds Amiboshi a mind-erase drug through a kiss in episode 40. In an author's note in the manga, Yuu Watase claims that she didn't mean for that particular incident to seem so sexual. Suuureshe didn't.
The revelation at the end of Chrono Crusade that Chrono and Aion are twins hasn't stopped some fans from shipping them. The fact that Aion really does seem to have a brother complex doesn't help dissuade them.
One of the fanon yaoi pairings in The Prince of Tennis involves a set of twins: the Kisarazus, namely Atsushi from Saint Rudolph (short hair, a red bandanna) and Ryou from Rokkaku (long hair, a white hat).
There are a few fanfics that pair up opposite gender twins Android 17 and Android 18 from Dragon Ball Z12 (Both of these stories are NSFW.)
Many fans pair Vash and Knives from Trigun. To be fair, it is strongly implied that Knives has one heck of a brother complex towards Vash. The first big confrontation between Vash and Knives at the very end of the first Trigun manga really looks like a metaphorical rape scene complete with grotesquely obvious phallic symbols. Which wasn't even the most disturbing thing about this scene in the first place.
For the fangirls and morbidly curious, it's in Trigun #2 (not Tri Max 2)
The Kongo brothers pairing in Eyeshield 21 fandom has a pretty good following. Agon's a huge narcissist and Unsui is very dedicated to his younger brother. It's not hard to imagine it being a little more then brotherly...
Twincest runs rampant in the Blue Exorcist fandom, with the Rin x Yukio pairing taking up almost 50% of all online romantic Fan Fiction (and normal boy x girl pairings less than 25%). Interestingly, there is no hint of yaoi in the actual series.
Ryan and Sharpay Evans from High School Musical have a suspiciously close relationship, what with singing love songs to each other and playing romantic leads opposite each other. Plus there's that video of "I Can't Take My Eyes Off of You", a song in which Troy, Gabriella, Ryan, and Sharpay all sing — Troy and Gabriella (established couple) sing to each other and Ryan and Sharpay... also sing to each other. Sample lyrics: "I can't take my eyes off of you/ I know you feel the same way too" and "You never really know what you might find/Now all I see is you and I/ You're everything I never knew that I've been looking for."
"If Sharpay could figure out a way to play Romeo and Juliet at the same time, her brother would be out of a job."
After Rowling told the fans what happens to Luna, there's been a bit of twincest involving her sons Lorcan and Lysander.
The Firebird's Son Trilogy plays on the Fred and George question; in it George is female and, while they are quite stated to be in a sexual relationship with Angelina and Lee, several characters have commented that the twins mother fears they would run off together. The fact that they are hyper sexual and extremely perverted (with Georgina confirmed to have had sex with Angela, and presumably Fred and Lee in foursomes) gives credence to this theory.
The twin sons of Elrond, Elladan and Elrohir and (to a much smaller extent) Elrond and his twin brother Elros are frequently paired in Lord of the Rings fanfiction.
In Xenosaga, Albedo is revealed to have been the conjoined twin of Junior. And let's just say that Albedo... was really obsessed with him, to the point where the thought of being unable to stay with him forever - among other things - drove him mad.
The Twins from Superjail! have somewhat of a fanbase that interprets them as incestuous based off their narcissism and closeness to each other. Certain lines (such as their "date" joke from an early episode) fueled the shipping, although the vague porn incident that traumatized them has been used by supporters and detractors of the ship to prove or disprove the dynamic being canon.
If Naruto or Harry Potter have twin brothers, they will generally be asses. If they have twin sisters...its twincest.
Films — Live-Action
Narrowly averted (at least according to Fanon) in Star Wars: the viewers (and the characters themselves) are initially led to expect a love triangle between Luke, Leia and Han, but then it is revealed in Return of the Jedi that Luke and Leia are actually twins.
However, the novel Splinter of the Mind's Eye was written and takes place before their relationship was revealed and so features romantic scenes with the two, as well as a scene in which Luke openly admires Leia's body. Particularly her "seal-curve hips". Apparently Splinter of the Mind's Eye was George Lucas's plan B if Star Wars wasn't a big enough hit to make The Empire Strikes Back, and would have been developed into a movie had there been no money to make the trilogy as hoped. At this point, he had not decided whether or not Luke and Leia would be family or a romantic item, but with all sources taken into context it looks like a pretty clear case of Abandon Shipping.
Marvel Star Wars, which also started production long before Return of the Jedi, also features a great deal of subtext early on. Notably there are many, many instances between The Empire Strikes Back and Return of the Jedi in which either Luke or Leia were trying for a Relationship Upgrade when they were interrupted by something or other. On a meta level, it was because the writers knew Han would come back and show whether "I know" meant reciprocation, but looking at it today makes it seem like the Force knew these two were twins and contrived to keep them from sleeping together.
The Yuuzhan Vong pantheon includes Yun-Q'aah and Yun-Txiin, sibling deities whose domain includes "love and hate and all things opposite". Their usual titles are either "The Twins" or "The Lovers". Possibly they either aren't twins or aren't lovers, and the Vong just have really confusing mythological canon. It happens. (For bonus points, a psyop plants a non-humorous meme in the Vong that the Jedi twins Jacen and Jaina are avatars of the twin gods...)
David Cronenberg's 1988 film Dead Ringers features some creepily sexual scenes between Jeremy Irons and Jeremy Irons as twin gynecologists Beverly and Elliot.
Based on the novel Twins, later re-released as Dead Ringers.
Definitely not to be confused with the Schwarzenegger and Devito vehicle.
Though that film surprisingly does have some eyebrow-raising twincest moments.
Julie Taymore's film version of Titus Andronicus portrays Chiram and Demetrius this way. They're just as touchy-feely with their mom, too. It's okay, though, because Anthony Hopkins puts them in a pie.
Hellboy II: The Golden Army. Okay, the twincest doesn't actually happen, but boy, does Prince BadassBishōnen Faerie want it to. It could be inferred that it had happened in the past. Considering that anything felt by one is felt by both, it's likely any masturbation experienced by one would automatically be shared with the other.
In Euro Trip, Jenny and Jamie get trashed on absinthe and make out (neither realizing who the other is). They are appropriately horrified when they open their eyes, and the next morning.
"Oh, here it is. Bratislava. Hmm. Capital of Slovakia. Oh, here's a fun fact: You made out with your sister, man!"
Not-so-subtly◊ hinted at in Brothers of the Head with conjoined twin brothers.
Billy Zane and Gina Gershon's characters in This World Then the Fireworks.
Cruel Intentions 2 had a scene where identical twin sisters make out in the shower. This film was originally produced as the pilot episode for a Fox TV series called Manchester Prep, but the series was cancelled before broadcast and so the pilot was "spiced up" for video release. One can safely assume this scene wasn't in the TV version. Rather ridiculously, in the scene the girls claim to only be cousins. This is one of only a handful of sexual scenes involving real-life siblings to appear outside of porn; as such, not only is it an R-rated example of Getting Crap Past the Radar, but it might be an example of the radar being turned off completely.
Stranz and Fairchild Van Waldenberg in Blades of Glory. The closeness of their relationship is a running joke throughout the movie and they even share a kiss at the end.
In the 1969 British comedy Some Like It Sexy, a guy gets into a threesome with twins (played by former Playboy playmates Mary and Madeleine Collinson, best know for the Hammer HorrorTwins of Evil). After a little while he realizes the girls are paying more attention to each other than him, and when he moves away they don't even notice. He leaves and the girls continue without him.
In Spencer's "The Faerie Queene" (1590) the giantess Aregant commits incest with her twin brother BEFORE THEY ARE BORN.
Cersei and Jaime Lannister in A Song of Ice and Fire. An unusual example of an incestuous relationship in serious fiction, as their affection for each other is depicted as genuine, unforced by either partner, long-term, and stable. (That affection is now rapidly waning, however.) Jaime is revealed to be true father of Queen Cersei's children, thus making them false heirs to the throne. This eventually is one of the main causes of the following civil war.
Goes from pretty nasty to very squicky when it's revealed that they were into each other even as little children — a servant walked in on them doing something sexual when they weren't even ten years old. Cersei even implies that the bond went back to birth, seeing as the two shared a womb and Jaime came out holding onto Cersei. This squicks the everloving fuck out of Ned Stark.
Of course, after three centuries under Targaryen rule, one wonders why the Seven Kingdoms even bother with the incest taboo in their ruling class...
Implied in Edgar Allan Poe's Gothic horror story The Fall of the House of Usher. It was damn near outright stated, the family had a history of incest, sooo...
There's a rather bizarre example in Fifth Quarter by Tanya Huff. The soul of Bannon is forced to share the body of his twin sister Vree when another man steals his body. Vree goes after her brother's body, and when they find it, they devise a plan that Vree will distract him by seducing him, and Bannon will "push" the interloper out his body. Partway through, Vree realizes that Bannon is more interested in seducing himself than in getting his body back, and calls it off.
Twins Charles and Camilla from Donna Tartt's The Secret History are revealed to be sleeping together towards the end of the book, although it is hinted at earlier in the book.
In Gary Jennings' The Journeyer, Marco Polo has a pair of twin lovers. It doesn't end well.
Ørjan is a shipper of Trine/Tore much to their and their friend Erik's annoyance, he constantly prints out lists of where siblings can get married.
Twins Estha and Rahel in Arundhati Roy's God of Small Things.
Shelley and Kipling in Judith Hawke's Heart of a Witch get into a very complicated relationship that was hinted at from the beginning with Shelley's possessiveness of her brother. Even more weirdly, this eventually turns into a threesome with Snow, the leader of the coven that they join.
In P.C. Hodgell's Chronicles of the Kencyrath series, Highborn Kencyr families used to practice sibling and cousin marriage extensively in order to breed Shanir (individuals manifesting powers of their triune God). Twin marriage was especially valuable for this, since it reinforced traits so strongly in offspring. Since a married pair of twins betrayed their people to the Big Bad and killed off two-thirds of them a few thousand years ago, it's fallen somewhat out of favor, but families that stick to the Old Ways still only breed with others in the extended family. The heroine and her twin brother are, by book 4 (To Ride a Rathorn), having very powerful erotic dreams together and some highly sexually charged meetings, and Word of God says that they have Single-Target Sexuality and will end up together (although not for a while; they have a lot of angst to get past first, especially brother). It's notable that some of the results of constant incest seem to be prevalent in the Highborn; they're dying out, fertility is down, childbed mortality is high, and some families are getting rather strange indeed.
Children of Dune has some pretty heavy overtones about Leto II and Ghanima's relationship at times. From Leto II's own mouth: "Let us pass puberty living in the same house, and all your taboos will be as a red rag waved in the face of a Sandworm." Perhaps mercifully, he undergoes transformation into a sandworm-like creature prior to puberty, thus removing this pairing from consideration.
... Of course, that doesn't stop him from marrying her later. But both of them are in complete agreement about the fact that there will be no weirdo hanky-panky going on, and that all her children will be bastards sired by a Corrino survivor, specifically the direct descendant of the emperor Leto's father deposed. (Though they're in love anyway, so, good for them.)
Not to mention that one of the reasons Leto transformed was to avoid the certainty of being maneuvered by the Bene Gesserit into this. The Bene Gesserit themselves view this "ship" as the only way to get back control of their Kwisatz Haderach program after Jessica sabotaged it by giving birth to Paul.
Stalking Tender Prey by Storm Constantine has, within the first few chapters, a pair of twins who, because they are hybrids of human, and a very oversexualised variation on the Nephilim, and the only two in the area, and therefore can only express themselves to each other get into twincest an awful lot. They're also Half-Identical Twins though this is not out of prudishness on the authors part, as one of them later turns out to be massively bisexual, and, lets be fair, most of the main characters do bedhop with most of the other main characters. In fact the main antagonist, as THE fallen angel, effectively satan pretty much gets to bonk 98% of the main characters. Repeatedly.
Incest is a common trope across all of Storm Constantine's work. Half-Identical Twins Valraven and Pharinet Palindrake are lovers in The Chronicles of Magravandias. Pharinet justifies this by claiming that they are one soul in two bodies and that they are the last of a great heritage. They almost conceive a child, but Pharinet miscarries.
It's really, really difficult to read the opening few hundred pages of Polgara the Sorceress without reading some seriously odd romantic undertones into the relationship between (technically) non-identical twin sisters Beldaran and Polgara: not only does Beldaran have her sister completely wrapped around her adorably girly little finger, but they're apparently sharing a bed well into adolescence whenever Polgara hasn't run away to live in a tree, and Pol's explicitly possessive anger when Beldaran gets engaged would be a little freakish in its intensity even coming from an actual romantic rival. (Technically technically, they are identical, making it even more fun. The physical difference or two comes from sorcerous manipulation in the womb by the mother gearing them towards their different destinies—Polgara was going to be a sorceress and Beldaran was going to be the queen of Riva. Apparently queens should be blondes and sorceresses should be brunettes.
In Gene Wolfe's Book of the New Sun, twins Agia and Agilus are found to have an incestuous relationship when Severian enters Agilus' cell the day before his execution, only to find the brother and sister nude and huddled together. Agilus admits, "Everything you suspect is true," the implication being incest.
Elizabeth Lynn's The Dancers of Arun features male/male twincest between Kerris and Kel. Oddly, it's portrayed as wholly positive.
In Robert A. Heinlein's Time Enough for Love, protagonist Lazarus Long relates a story about a twin brother and sister he encountered who were unrelated to each other through a remarkable feat of genetic engineering, and were induced from childhood to form a sexual bond with one another for breeding purposes. As they are slaves, he buys them, frees them, and sets them on the road towards being independent human beings, but they still end up marrying each other and pumping out babies.
A probably nonconsensual example: in The Acts of Caine novels, in one book, Berne is revealed to keep twin sex slaves, one male and one female. This being Berne, if you think he wouldn't order them to have sex with each other, you plainly haven't read the books.
In Death: Rudy and Piper Hoffman from Holiday In Death are brother and sister, twins, and... have at a least a sexual relationship with one another. Rudy has admitted that they have tried not to act on the attraction between them and be with other people, but they were miserable. Miserable because they are attracted only to each other. Hoo boy.
A twist on this trope: The two protagonists of Alistair Reynolds' House of Suns are among thousands of both-sex clones of one woman. They are genetically programmed to be space-explorers, and all are supposed to meet once every 200,000 years to share information. Their progenitor forbid inter-clone romantic relationships and, while sexual encounters are said to occur occasionally, the protagonists' long-term consorting is strongly rebuked by their fellow "siblings."
Emmeline and Adeline's relationship in The Thirteenth Tale has very strong incestuous overtones.
Rika and Saxon Hyde from Michael Slade's Ghoul engage in this as teens. To amp up the Squick, they're the product of Brother-Sister Incest themselves.
Violet and Verbena Pollard from Dean Koontz The Bad Place are twins and very... close. But it seems this kind of thing is par for the course in the Pollard family.
Thomas Mann's novella "Wälsungenblut" features Siegmund and Sieglind, twins who seem to go everywhere holding hands, and spend a good chunk of the story attending a performance of Wagner's Die Walküre. Everything goes as expected.
Nikolaj Coster-Waldau: It's a great phrase! Twincest...
In the final episode of Arrested Development, Michael's twin sister Lindsay admits that she's attracted to him. However, this is only after the revelation that Lindsay is adopted.
Admits?Her almost immediate reaction to learning they're not related is to try and convince him to marry her.
And then later Gob comes on to her when he learns about this. She punches him.
It's impossible to watch Maya and Alejandro Herrera in Heroes without thinking about it. After all, Maya killed everyone but Alejandro and herself during her twin brother's wedding after she learned that his bride to be was cheating on him...
House points out that two identical female twins who are both fighting to join his team are "distracting to every man and lesbian in the room".
There was a pair of fraternal male twins that the main characters used to go to school with on It's Always Sunny in Philadelphia that were sexually involved. Turns out the entire McPoyle family is one large, ingrown incestual orgy. It's just about the creepiest thing on television.
In "Who Got Dee Pregnant," the gang (minus Dee) think that Dennis was the one who had sex with Dee at the Halloween party (due to Dennis switching costumes with Charlie and Dee with the girl Dennis was trying to hook up with). Dennis is throwing up when they confront Dee, with them telling her she has to go to Mexico to get an abortion- which she states none of them are the father and although she pulled Dennis into the back room, they didn't have sex.
Jack from Will and Grace has, as his life's ambition, to one day meet and have sex with a pair of male identical twins. Will once actually invited over a pair of gay identical athletic twins just to make Jack jealous.
Katie and Emily Fitch from Skins. Moreso Katie than Emily; she states that Emily is "hers" and Naomi (who is romantically involved with Emily) could never love Emily as much she does. It doesn't help that at one point during the end of the twins' episode in series three, when Emily is confronting Katie, someone in the crowd yells out that Emily should kiss her.
And in the book, the Fitch family are on holiday in France, and Katie steals Emily's phone so she and Naomi can't stay in touch.
In an episode of Medium, some characters come across a porno that features this.
Definitely played with by John and Edward in X-Factor, such as their routine to "Oops I Did It Again", which involved a full re-enactment the talky bit between Britney and her love interest from the music video ("but I thought the old lady threw it into the sea at the end of the movie").
For anyone masochistic enough to want to see them in action, here is the aforementioned performance.
Beastmaster had a fraternal twin example in its first season.
Tao: It says you were born in the same place... and at the same time. Gem: You are the first to understand. Tao: This has to be wrong. Nye: It's not wrong. Gem: We know our own story. Tao: But it says you were born of the same mother. (crickets)
In Degrassi Takes ManhattanFiona drunkenly kisses Declan at a party in front of everyone to make Declan's girlfriend jealous.
It took twelve seasons, but SVU finally went there in the episode "Bombshell": guest-star Rose McGowan played a con artist/swinger who was swindling people with her twin brother—and also sleeping with him because they "share a soul." Elliot even calls the trope by name.
Implied when the Veronicas, a band consisting of twin sisters Lisa and Jess Origliasso, sing "All About Us", a song that, while written by them, was made famous by t.A.T.u., apparently Schoolgirl Lesbians.
The story goes that the song was originally a demo done by the Veronicas; when they couldn't fit it into their lineup, they handed it over to t.A.T.u. Which is not to say that the Veronicas are not twincesty as hell — see their propensity for taking ridiculously inappropriate-for-sisters pictures. Seriously. Not to mention their musicvideos. No...THIS video. Also this is the official video for the first link...and a little more twincest sexy
At least once when performing live Bill has dropped to his knees in front of Tom, while singing a song supposedly about Tom and a groupie. More specifically, he sang "Ich will mit keiner ausser Dir." Which roughly, in context, translates to "I don't want to do it with anyone but you." He ALWAYS sings that part to Tom.
Unfortunately, though, "keiner" denotes a female "anybody" (the male form would be "keinem"). Which either rules out the homosexual context or says things about Tom we'd better not go into.
That line has also been translated as "I won't do anything without you" as in "without your conset, i.e. not going to rape you" which makes more sense as "Reden" is about a one-night stand. Not sure what effect that has on the implications.
"I'm ready to fall, I'm ready to crawl right on my knees to know it all." Practically on his knees in front of Tom while admitting he's okay with being down there? Oh Bill, learn some subtlety.
This seems to be the gist of the relationship between the Kagamine twins of Vocaloid, at least among the fandom. Though they look like twins, Crypton originally presented them as such before making them "mirror images," then finally telling the fans to decide for themselves.
Myths & Religion
In the Feri Tradition (one of the Neo-Pagan movements), there is a pair of twin gods who basically share all aspects of themselves with each other (and not just in that way), or so claims The Other Wiki. They also don't have a fixed gender.
In the Völsunga saga, Sigmund is seduced by his sister Signy in disguise, as she believes only in this way she can conceive a son that is strong and loyal enough to help Sigmund avenge their brothers and father on their killer, Signy's treacherous husband Siggeir.
Egyptian Mythology had Osiris and Isis, as well as Set and Nephthys, the four of which were all twins who fell in love while still in the womb together.
Expect this in any pictorial featuring twins in the less classy porn magazines.
In 2003, Shane and Sia Barbi sued Hustler for unauthorized use of photos showing "intimate sexual acts between the twin sisters".
This trope (actually done sympathetically) is the centerpiece of John Ford's ever-controversial Tis Pity Shes A Whore. Unusual in that the creepiness of the setup comes from a) how similar it is to a regular love tragedy, and b) how its lack of acceptance leads Giovanni to snap and rip out Annabella's heart.
This trope (along with a helping of Creepy Twins) is the premise of Marina Carr's play Portia Coughlan. Portia is haunted by her twin brother Gabriel, who drowned himself as a teenager; the fact that she's still obsessed with him fifteen years later (and that Gabriel is referred to as "unnatural") hints heavily towards this, so it's no surprise when she reveals that their relationship was sexual. It's a bit more surprising when her aunt reveals that Portia's parents, too, are half-brother and sister, though neither of them know it.
The entire plot of the rather strange — and very sexual — British queer play Over There. The two-man cast consists of the identical twins Harry and Luke Treadaway, and I can only say that the two certainly aren't uncomfortable with each other.
Part 2 of Richard Wagner's operatic Ring of the Nibelung cycle, The Valkyrie, has Siegmund and Sieglinde: Twins separated at birth, who meet as adults and fall in love which each other, finally giving birth to the legendary hero Siegfried.
Visual novel Yosuga no Sora's most famous pairing, Haruka and Sora, are twins. The fact that Sora is implied to masturbate as she imagines having sex with her brother BEFORE the events of game even start clearly doesn't help...
Metal Gear Solid, being chock full of Ho Yay moments, certainly didn't shy from playing with this trope. Liquid Snake strips Solid Snake's shirt off while the latter was unconscious, and there was his desperate need to prove that he was the dominant brother. Naturally, it's kicked up to overdrive in the remake. It also certainly didn't help that Liquid was a Large Ham and his voice actor, Cam Clarke, seemed to have an uncanny ability to make the word "brother" sound very erotically incestuous.
Liquid does blatantly say that part of his motivation is that, 'in Nature, family members don't mate with each other, but they help each other to survive'. Bad biology aside, it's pretty easy to overthink how that would justify him killing his brother instead of 'helping him to survive'
In Metal Gear Solid 4, when Snake and Liquid are having their final fight, if Snake can't shake liquid off, Liquid will kiss Snake's cheek. However, at the time, Liquid is possessing the body of Revolver Ocelot so YMMV on the level of twincest. Subverted in the fact it's really just Revolver Ocelot impersonating Liquid. The matter is further complicated considering Ocelot had a crush on Snake's identical clone "father", Big boss, which adds Love Father, Love Son into the whole mix. The whole thing can be rather...confusing. Then again, it wouldn't be Metal Gear Solid if it wasn't.
Fatal Frame II: Crimson Butterfly, with their whole "becoming one with your other twin". One of the key plot elements of the game is the fact that the village use to perform rituals in which the older twin would strangle the younger one, who would then become one spiritually with the older twin and then they'd get to be togeetheeer foreeveeer. This whole "I want to be with only YOU, -insert other twin's name here-!!" thing happens with prettymuch all of the Twin Shrine Maidens depicted in the game, including but not restricted to Azumi and Akane Kiryu, the protagonist Mio and her sister Mayu, and the antagonist Sae with her sister Yae. Sae seems to be the one with the most weirded-out obssession with her sister, since she seems to all along have fantasized about the day that she'd get to be strangled by Yae by the end of the game you find a crystal left by her that is pretty much along the lines of "Oh Yae, kill me, KIIILLL MEE" like she's getting turned on by the idea.
Not to mention in the promotional comercials, Mio and Mayu are shown in a number of amusingly suggestive poses together. The strangling pose Mio puts herself in above Mayu at the end of the game in Normal mode is also very suggestive.
Aaaaand some official art has them sitting together or swiming together in bikinis. Very...◊ very◊ closely.◊
In King's Quest III, after Alexander frees his twin sister Rosella and talks to her, he seems attracted to her in a not-so-platonic way. Of course, they were also Separated at Birth.
The same reason was why a lot of Phoenix Online players were scratching their heads at the opening scene of The Silver Lining (a Fan Sequel to the series). Sure, it makes logistical sense to keep the comatose twins in the same room where their respective spouses can act as a final line of defense... but the same bed?! Um... No Yay.
Lucjan and Myaja, a brother-and-sister fighting team in Dragon Age: Origins, state that "everything they do, they do together." When the PC asks them if that means everything, Myaja asks if he wants to find out.
In The Sims 2, if you turn off the switch that prevents incest relationships, twins fall in love very easily, without intervention from the player. Mechanics-wise, it is probably due to their interests and personality traits being alike, therefore making the "perfect" match as far as the game is concerned.
Crossing over with the Screw Yourself trope there's Morrigan and Lilith Aensland from Darkstalkers. Lilith is a shard of Morrigan's soul but she has been given a separate body. Being Horny Devils that survive on pleasure sorta justifies their relationsip.
Fraternal twins Lucy and Fisk in Better Days. Whether it happened more than once is a matter of debate. Lucy at least continues to have more than sisterly feelings for her brother well into college.
Given that there are several early Naylor porn-folios centered around Lucy and Fisk, it seems likely that they were at least originally intended to have a long-running relationship (as their counterparts in Badly Drawn Kitties did until his departure)... then again, those portfolios are stricken from the record as far as Naylor is considered, mostly because the copyright notices affirm that Lucy is not his original creation.
Rick and Kate in Concession, though that is not by any means the only way they are messed up. Also Joel's connection to his sister's ghost seems to border on this, though she died too young for anything to really happen.
The twin lawyers in DevilBear have a strong vibe of this going. Given that they are infernal servants and borderline Horny Devils, it's not unexpected.
Subverted in Oglaf. While trapped in his Lotus-Eater Machine, Ivan's love interest says she's delivered twins and shows him...two minature copies of herself making out. Rather than be a turn on, it's the sight that makes Ivan briefly realize it's All Just a Dream.
Succubus twins Giselle and Alizee in Leth Hate, although not voluntary on Giselle's part.
Heavily implied of the Subway Masters in Black Adventures (the Subway is not affected by a power outage because "it runs on our bruderliebe"), and accidentally the case for Black's crush on White when she's revealed to be his twin. In the latter case, it's used for Ship Sinking rather than titillation.
Whilst nothing explicit has happened (Yet?), Survival of the Fittest V3 characters Lenny and Elizabeth have a whole lot of Subtext in their relationship, prompting a lot of "speculation" amongst members of the board. It isn't clear precisely what relationship the pair have, but it is implied it goes further than being simply siblings. Recently the handler more-or-less addressed this issue, quoting a Fanfic in this first half of one of his posts.
In Tea, Biscuits and Incest, Makayla finds out that her ex-boyfriend Jayden is her long-lost twin. Doesn't help that he's also sleeping with their father.
A vast thread on the RPGNet forums, covering creepy gamers, had a poster effectively troll the discussion by promising to elucidate on a story of creepy gamers that involved twincest. Although there were several Crowning Moments of Awesome and Funny involved on the thread as it was, the rumoured story turned out to be disappointing, though it did get an airing.
In "The D'oh-cial Network", an episode of The Simpsons, the Winklevoss twins make a brief cameo, in which they compete against Patty and Selma at the Olympics. The scene ends with Cameron and Tyler making out.
Andy and Ollie Pesto of Bob's Burgers are prepubescent but definitely headed in this direction.