@greyhud90 it happens ONLY WHEN THEY HAVE CHILDREN. If not, what's the problem? (You could even argue that having kids is morally wrong even when you and your partner is perfectly healthy and not blood-related).
Anyway, you know this is just for the added spice. The author challenges the idea of incest=bad, even when a) they're not aware of each other bloodline, b) most importantly, they're in love with each other, and see how it works out. Could be good, could be bad, but at this point in time, it doesn't deserve all that one star rating.