(chiefly fandom) Incestuous sexualrelations between twins, or siblings who look very similar.
2007, Rachel Kramer Bussel, Best Sex Writing 2008These pieces you might very well be able to relate to even if you've never been horny for twincest, had an affair, or been a black man...
2013, John Lancaster, "When did you get hooked?", London Review of Books, vol. 35, no. 7:In the course of one of his climbs, he hears adult voices through a high window, goes to investigate, and comes across Jaime and Cersei energetically engaged in (to use a neologism popular with fans) twincest.
