Brothercest
A fussion of brother and incest it's a genre of fanfiction that involves two brothers in a sexual and/or romantic relationship. Although a subgenre of slash fiction because it's about a male/male couple, it's not widely accepted in slash fandom
One of the most well-known brothercest pairings is Sam and Dean Winchester from the tv show "Supernatural", AKA Wincest. Other common example is Petrellicest, involving the brothers from tv show "Heroes", Nathan and Peter Petrelli.