I'm with dudemon here; it's not like not having the children - i.e. not giving them life at all - is more moral.
There are inherent risks with a lot of things we do. Shouldn't make them illegal, or even morally objectionable, when there's nothing they can reasonably do about it. I'd sooner make drugs/alcohol/smoking illegal for the same reason, except this couple can control their intake. They can't control their genes.
...not an endorsement of incest, btw. Just commenting on this case.