“There’s nothing in the Bible about not marrying your cousin, yet we have a taboo against that. So it’s not all from the Bible.”
After 1,400 years of first cousin incest, Muslims have accumulated a range of genetic deficits due to inbreeding. ‘Course, they were not much to start with.
first cousin is bad enough
but sister and brother is horrific
they have the same dad, they are bro and sis
Two words: Habsburg dynasty.
Bear in mind that Abraham and Sarah were brother and sister.
I think the Muslim case is a special one because not only do they allow 1st cousin marriage, the actively encourage it. The only men that a woman is allowed to have normal interaction with that she can marry are her cousins, uncles, etc, so it’s a natural outgrowth of their restrictions on women. In a normal society, where the practice was simply allowed, but not encouraged, you probably wouldn’t see the kinds of recessive accumulation you do in the Muslim world.