Incest between twins
Incest between twins or "twincest" is a subclass of sibling incest and includes both heterosexual, homosexual and lesbian relationships.
In Asia
In traditional Balinese culture, it was common for a set of twins of the opposite sex to marry each other, since it was assumed that they have 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.Incest was commonplace in Southeast Asian creation myths which prominently featured twin or sibling couples. In these stories, the brother usually wooed and wed his sister, who bore his child or children, but on discovering that they are siblings, they are often forced to part.
In Europe
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.The theme also appears in English literature, such as the incest between the twins Polydore and Urania in Delarivier Manley's The New Atalantis.
One supposed case of incest between twins, in which twins who were adopted by separate families as infants 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. 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 due to the Westermarck Effect.
In fiction
The fantasy fiction series A Song of Ice and Fire describes a secret relationship between Jaime and Cersei Lannister, who are fraternal twins. Within the setting of the books, various historical characters from House Targaryen were married to their siblings.The Japanese series Yosuga no Sora features heavy depiction of romance and sex between twins heterosexual Haruka and Sora.
In the 2011 series Law & Order Special Victims Unit: season 12 episode 19 "Bombshell"- Cassandra Davina played by Rose McGowan was involved in a sexual relationship with her twin brother telling Detective Stabler her and her brother came into the world together and that they were each other's soul mates.