Whatever floats your boat, man. Personally I couldn't ever bang my sister, but if someone wanted to bang theirs then whatever.

The only one problem I can see is having children, but I don't think there's anything really notable unless you inbreed for like ten generations or something. But I'm not expert.

Oh and then there's the whole society looking down on it thing. But I mean it's not like that's limited to inbred children. Homosexuals and Transgenders can face bullying and discrimination as well while growing up. As well as those raised by two men or two women.