I think incest should be legal. If they're both consenting adults then there's no issue in my opinion.
If the concern is of children being born with disabilities, then one ought to want to bar them from having children, not from having sex. However, I don't think the risk of genetic abnormalities is particularly high, being comparable to a child being born to an older woman.
What we have currently seems to be people's emotional reactions having been enshrined in an old law, rather than a law that's a result of rational, consistent and fair consideration of the issue and individuals' rights.