It's an interesting question. Initial reaction is to recoil in horror at the thought, but then i stop to think - what if i had a very attractive sister my own age? I can look at my friend's sisters and find them sexually attractive (IE not finding it weird that it's the same "gene pool" as the friend in question, so to speak, if that makes any kind of sense) so would i be able to feel the same way towards a biological sister who was undeniably attractive? The answer is... i don't know. I'm an only child, so i have no idea what it's like to have siblings. The stereotype is that siblings never get on and always argue (the OP said he was "close" to his sister but despises her for example. How does that work?) but i'm almost certain there are far more loving relationships between siblings than that, and i suppose it's about blurring that line between love and "love", which might be a dangerous notion. Though, thinking about it, when you're with a new partner you might be nervous about being judged about each and every thing, but if it's your sibling, they already know everything there is to know about you. You'd be able to be totally relaxed around them.
Morally, i would have to say that it is wrong, because of societal context. We've been taught it's wrong and that is that. But i still find it curious to mull over the question. After all, our reaction to incest now is the same as what was society's reaction to homosexuality not that long ago.
And for those talking about birth defects being the major problem, what if it was homosexual incest? (2 guys or 2 girls)? That point no longer becomes valid.
