Do we forbid people who have high chances of passing on diseases to their children from having children or marrying people who might increase that chance? Seems to me that we make them aware of the high chance they have of passing on genetic disabilities to their children - genetic counseling - and then let them do what they choose to. While I am morally opposed to incest, I'll hypothetically enter the conversation to ask: What is the difference between two none-related people with high chances of passing on genetic disabilities to their children and two consenting siblings with high chances of passing on genetic disabilities to their children? In both cases, they know the risks in advance - yet we allow one set of consenting adults to procede as they wish and we forbid the other set of consenting adults to do so...all based on the whims of our societal morality.
It seems to me that the argument against incest doesn't hold much water (legally speaking) if we are, as a society, legally pursuing a course of saying what happens between two consenting adults is the the business of no one but those two (or three or four or ???) people. Here's where legal precedent becomes a very tricky thing.
You clearly don't know the odds of brothers and sisters having f'd up children. It's about a 19 in 20 shot with a 1 in 20 margin of error. For regular people it's like I don't know 1 in 6 to 1 in 25 depending on who you believe. And if you have good genetics and aren't engaging in drugs and alcohol and are of proper age then I think it's way better yet.
I think it's very dependent upon the individuals involved. It's hard to come up with an accurate number for odds, as I understand things. A lot of it has to do with recessive genes; if two random people have a child, the odds of both having the same dangerous recessive gene and passing it along to their child is less than for close relatives, they are more likely to carry the same gene and therefore more likely to pass it along.
So, it's more dangerous for siblings or other close relatives because if there is a dangerous gene in the family, both are much more likely to have it, but if the siblings have good genes it's not much more dangerous than for anyone else, if at all. When family interbreeds over generations would be where the real problems occur.
This is all my layman's understanding and not based on any extensive knowledge or research. I certainly wouldn't expect anyone to just take my word for it, I'm not entirely certain of how it works myself.