Citations:twincestual

Adjective: "(neologism/rare) Pertaining to or engaging in twincest."

 * 2012, Kelly West, "True Blood Season 5 Premiere Watch: Turn! Turn! Turn!", Cinema Blend, 10 June 2012
 * takes their relationship to an icky level, though not entirely unfamiliar to HBO (See also: Game of Thrones' twincestual Jaime and Cesei Lannister).
 * 2013, Christopher Orr, "Game of Thrones: A Feminist Episode, a Gay Episode, or a Dull Episode?", The Atlantic, 7 April 2013
 * And Brienne and Jaime have two of their best scenes together to date: his enunciation of a decidedly liberal (and entirely self-serving) philosophy of sexual freedom — straight, gay, twincestual — and her rather persuasive drubbing of him on the bridge.
 * 2015, Kat Rosenfield, "A THERAPIST EXPLAINS WHY EVERYONE ON ‘GAME OF THRONES’ HAS SERIOUS ISSUES", MTV, 30 April 2015
 * Jaime and Cersei Lannister are the hottest twincestual couple on television, but that’s not even the biggest reason why they’re a bad, bad match.