>>5
Oversimplified, and the truth is scarier.

Chances of some of the nastier, more lethal birth defects increase as a woman ages. For example, if a woman is over 40, she has about a 1 in 25 chance of giving birth to a child with Down's Syndrome (and Down's children rarely live past the age of 25 or so, it isn't just their faces and their brains that come out messed up, it's their hearts too).

And when first cousins marry, the risk of severe birth defects is 5x that of the baseline population--which is why it is banned in most of the US. I have seen no figures on brother-sister incest but it can only be more dangerous. When nasty recessive genes get reinforced on both sides, they come out to play and horrible things happen. The more closely related the parents are, the greater the odds of lethal recessive genes being combined from both parents.