Father/daughter for the win, every time. Mother/daughter and brother/sister (especially with younger sister and especially, especially with loli sister) are OK too. Mother/son is a huge turnoff to the point that I won't start most games that have other fetishes (like loli) in them that I like, just because I have to fuck the mommy. If it's skippable (as in, doesn't happen if I don't want it to) that's fine. There are some exceptions (like Such a Sharp Pain) where I will endure the mommy x shota for backstory purposes and because everything else is so damn hot.