I think resistance to changing the law on this is entirely societal, which shouldn't have an effect on law, but does. There are plenty of things people do that society may disapprove of in general (being a dick, most of the time) but we're legally entitled to do as long as we don't step on others toes too much.
I also think there are two issues at the heart of incest. The first issue is the same reason a lot of teachers get fired- it's the abuse of the position of power relationship. Making incest legal could allow for grooming by adults with a view to sexually abuse children when they're older (which would be perfectly legal as they're eventually of a consenting age). It could also lead to 'well, it'll be ok later on, so it's ok now' thinking. Pedophilia laws partially cover this, but incest laws double up.
The second, separate issue is offspring. Potentially, it could be legalised as long as it's safe/non-reproductive sex. The argument against that is that we don't make reproduction illegal for people with other disabilities; instead it's up to the individuals in question to consider the chances and difficulties that may be involved in bringing up a disabled child. I certainly don't think there should be a double-standard there.
Even amongst statutory non-consenting siblings, the most damage I've seen done has been the 'outing' of it, making a big fuss (whether via religious or bringing the law into it) and throwing the victim into the limelight. I think, at best, parents may strongly discourage it and educate their kids on it, much like you would with any precocious and pubescent child, but once they become adults, it's their responsibility to judge how society may judge them (if found out) and the issues involved.