Did you read Your citation? Because it doesn't back up your assertion. It's says that for for first degree relations (brother-sister or parent child) the risk of death or severe defect increases 31.4% over the general risk. It doesn't say what the general risk is, but I assume it is relatively rare, on the order of 1 in 1000 or fewer. So risk to offspring from siblings would rise to about 1.3 in 1000. I would hardly call that proof that sibling marriage results in defective offspring.