Generally, most people have good genes and bad genes.
But we have two copies of each gene, one from mom and one from dad.
And a lot of times, we get a bad gene from mom, but that is masked by the good copy we got from dad. Or vice versa. That's why genetic diseases don't happen a lot.
But, if two siblings have a child, then most likely the siblings will BOTH have copies of the bad gene, and therefore the baby will get 2 bad copies of the gene, and therefore a disease.

Most of the time, I think total autonomy is a good thing. Let people do what they want.
However, this is a terrible poser, because if two siblings have a child, chances are extremely high this child will have genetic diseases and therefore a horrible life.
In this case, it doesn't affect just the siblings whom had the kid, but also the kid itself...
I have to think about this...

"Words are useless, especially sentences. They don't stand for anything. How can they explain how I feel?"~Madonna, Bedtime Story

Not children, just legal relationships. Gay or straight.