That maybe sounds wrong, but if we even go the incest route, I don't think a slightly younger person or younger looking one would be the worst in such a game.
1. While I don't like it, they seem to do it far to young from what I heard during this age.
2. I suppose even something around 16 would have been okay for a game like that? I can't think of anything to bad in this situation, especially with the context the game would probably give it.
i don't know I might sound very wrong with my english trying to explain more complicated stuff with a quick thought.