> There's no evidence that sibling marriage results in defective offspring. It's simply the fact that we find the idea of siblings having sex to be icky
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.
Does this mean that marriages between same-sex siblings will be permitted?
It's simply the fact that we find the idea of siblings having sex to be icky. Which is the same reason for the prohibition against same-sex marriage.