To be serious and not joke, iirc there’s a psych phenomenon that describes sexual revulsion to people that you spent your formative years with as a mechanism of separating out relatives and potential partners even at the subconscious level. Weird stories of cousin fuckery and long lost sibling relationships abound because they didn’t actually grow up together despite the blood relation. And idk what brain folds effect the kink gene but god knows what’s going on with the people actually fetishizing incest for incest’s sake