Hoy en la tarde estaba buscando definiciones sobre lo que es twincest y eso y en wikipedia encontre esto:

En la sociedad Balinesa : En lo tradicional , era común para un conjunto de gemelos de sexo opuesto al matrimonio entre sí, ya que se suponía que tenían relaciones sexuales en el útero.

En europa: (mi traduccion) El incesto entre gemelos es algo sobresaliente en la mitologia germanica, y sus manifestaciones modernas, como Siegmund y Sieglinde en “Die Walküre” de Wargner. Los conflictos sexuales y el incesto entre gemelos es tan bien sobresaliente en la obra “Grotesque Literature” del autor aleman Kerstin Hensel.

En una revision de las literatulas centradas en homosexualidad e incesto entre gemelos, Ray Bixler concluye que ” más gemelos homosexuales del mismo sexo, si se crian con sus gemelos, no intentan nisiquiera seducirlos en la edad adulta.
Un caso de gemelos, que fueron adoptados cuando eran bebes por familias diferentes, se casaron sin saber que eran hermano y hermana, pero ellos no son el unico caso que ha habido.

Aqui les dejo el texto original, solo traduci lo que me parecio importante, espero que les sirva.

Twin incest is a prominent feature ancient Germanic mythology, and its modern manifestations, such as the relationship between Siegmund and Sieglinde in Wagner’s Die Walküre. 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.[3] The theme also appears in English literature, such as the incest between the twins Polydore and Urania in Delarivier Manley’s The New Atlantis. [4] Sexual conflict and incest between twins is also a prominent feature of the contemporary German author Kerstin Hensel’s “Grotesque Literature”.[5]
In a review of the scholarly literatures on twin homosexuality and twin incest, Ray Bixler concludes that “most same sex homosexual twins, if reared with their co-twins, do not attempt or even want to seduce them in adulthood”.[6] His study draws on Edvard Westermarck’s hypothesis that sexual desire is generally absent in relationships between members of a nuclear family.[7]
One case of incest between twins, in which a pair of 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.[8] The story was widely publicised in the British press, although its truthfulness was called into question.[9]