Levi Strauss explained it better, but it all gets down to kinship and territory. Growing families and shit. But there's nothing intrinsically wrong with it, really. >>97135115
That only happens after generations of incest. That is, if you fuck your sister, and you have kids, and then the kids fuck each other, it's likely they'll have more defects but that isn't always the case. Actually, incest happens all over nature.
It's natural for societies that thrive among themselves, like animals or royalty.