In traditional Balinese culture, it was[when?] common for a set of twins of the opposite sex to marry each other, since it was assumed that they had sex in utero. The standard anthropological explanation of this custom is based in explications of the conflicts between descent and affinity in Balinese society. Twin incest was a common feature of Balinese mythology. As in many other mythologies, the Balinese deities frequently marry their siblings without any of the incest-related issues faced by similarly-situated human couples.
This was commonplace in Southeast Asiancreation myths which prominently featured twin or sibling couples. In these stories, the brother usually wooed and wed his sister, who bore his child(ren), but on discovering that they are siblings, they are often (but not always) forced to part.
According to Tagalog mythology, Malakas ("strong") and Maganda ("beautiful"), the first humans on earth, were fraternal twins.
An old Japanese myth says that if two star-crossed lovers commit dual suicide, they are reincarnated as fraternal twins.
In European culture
Twin incest is a prominent feature in ancient Germanic mythology, and its modern manifestations, such as the relationship between Siegmund and Sieglinde in Richard Wagner's Die Walküre, and a feature in some Greek mythology, such as the story of Byblis and Kaunos. There are strong parallels between the Germanic portrayals of twin incest and those of the Balinese Ramayana, and some scholars have speculated an early Indo-European link.
In a 1983 review of the scholarly literature on twin homosexuality and twin incest, Ray Bixler concluded that "most same sex homosexual twins, if reared with their co-twins, do not attempt or even want to seduce them in adulthood". His study drew on Edvard Westermarck's hypothesis that sexual desire is generally absent in relationships between members of a nuclear family.
One case of incest between twins, in which twins who were adopted by separate families as babies later married without knowing they were brother and sister, was mentioned in a House of Lords debate on the Human Fertility and Embryology Bill in January 2008. According to the charity Adults Affected by Adoption, there had been other cases of this sort that had involved siblings. The story was widely publicised in the British press, although its truthfulness was called into question.
Czech identical twins Elijah and Milo Peters are male porn actors who work condomless, performing both anal and oral sex on each other in video performances since 2009. They consider each other to be both brothers and romantic partners, and report that they do not have sex with any men besides each other when they are not on film. 
The 1975 novel Gemini by Michel Tournier depicted the deeply sexual relationship between twin brothers Jean and Paul and their struggle to come to terms with the implications of their sexualized world consisting of only each other versus their individuality in the outside world.
The novel The Secret History also depicts a sexual relationship between twins Camilla and Charles Macaulay.
Incest (as "forbidden love"), especially between siblings, particularly twins, has regularly been featured in Japanese fiction, most visibly in mediums appealing primarily to youth and "otaku" such as manga, anime, videogames, light novels.
In the anime and manga Yosuga no Sora, fraternal twins Haruka and Sora develop an incestuous relationship at the end of the series, and its is later revealed that they moved overseas to find happiness, got married and are expecting their first child.
In the short film "Keith & Heath", two identical twin brothers (who are puppets) are the best of friends until a girl threatens to tear them apart. The film ends with the two brothers getting married and kissing, which turns them into humans (played by Jon Cozart) and chaos ensues.
Sexual conflict and incest between twins is a prominent feature of the contemporary German author Kerstin Hensel's "Grotesque Literature".
In the 2004 teen-comedy film Eurotrip, fraternal twins Jenny and Jamie (Michelle Trachtenberg and Travis Wester), tour Europe when they decide to have some fun and go to a nightclub. Drunk on absinthe, Jenny and Jamie French-kiss continually, witnessed by Scotty and Cooper, until the twins momentarily stop to see who the other is and are horrified when they realize what they are doing.
The word 'twincest' is used in the 2014 movie Gone Girl about the possibly over close relationship between the chief protagonist and his sister.