Just found out mating w/ siblings isn't particularly bad in any special way. It's bad in that it gives more chance for the expression of a negative phenotype but other than that, it's pretty okay. Society just looks down upon it.
I can answer that goodhen. Basically, it makes the gene pool weaker and the human race has found out that if they say that they hate it, there will be less incest and therefore less weak genes in the gene pool and therefore a stronger human race.