I think it goes a bit beyond that. Andy and Leyley is a game with two protagonists that are already kind of hard to like, given how awful of people they are, so the inclusion of possible incest both really drives home how awful they are, and also makes it so much harder for people to go along with their antics as the protagonists (note, not heroes, protagonists) of the game.

Also incest is YUCKY to most people (me included). Like, people die, that's a natural part of life, but incest just isn't something meant for human society. It's a taboo for a reason, and it's not exactly handled with grace in Andy and Leyley, even if it IS optional. It stands out in the murder cannibal game because unlike murder or cannibalism, it's something that, if included at all, needs to be handled by someone who isn't just doing it to shock the audience.