I think War of Heirs (rip) had a route where you could romance your first cousin as a male MC. You could still play it in the Dashington archive here, but I doubt the writer is ever coming back.
Bastard of Camelot, with the Dragon bestie being an RO for some reason. Still don't get why people requested that, or why Llamagirl fulfilled said requests. The game is already stuck in the childhood stage as it is, and we haven't even met all the LIs yet to have a rewrite adding another.
Okay so war of heirs is disappointing because with the way some of y'all talked about it i thought the wip already had the incest route but no it barely even got out of the metaphorical womb of IFs (A prologue and 1 Chapter)
Anyways IF authors like to dip into that mature themes nowadays but honestly I think most of em either hold back on the gas pedal because of purity culture or they only like the superficial themes like it's an aesthetic so we end up having mature IFs that's better off as shoddy erotica