Oh please. First, as DW said, that doesn't really work for cousins. Second, people marry every day after genetic testing predicts high risks of bad disease. Consenting adults, eyes open, what's the problem. If 40 year old brother and sister say they're in love, why should the state intervene? To protect a hypothetical unborn child? What if they promise to use birth control, and you test them periodically and abort any children?
I never understood the "rape and incest" carve out on abortions either. Why should abortion be allowed more for a consenting, adult woman who was involved in an incestuous relationship than a non-incestuous one?