The social policy behind outlawing this kind of behavior is larger than just the unfortunate biological side effects. It's important that there are relationships and community where sex is just out of the question. Sexualizing family life would lead to the ultimate breakdown of the family unit as we know it.
There are cultures where cousins are allowed to tie the knot, and cultures where having the same family name can be enough to ban a marriage.
What is seen as family life boundaries for sex is culture bound.
In lots of old cultures cousin relationships were favoured, because it kept money within the family. It did bring diseases withing families too, if that kept going on generation after generation.
Depends on where you draw the line. No culture has ever embraced siblings or parents.
And siblings or parents are off limits with many other living species too.
