It's not surprising at all they don't suffer abnormalities. I've said before that this is a fallacy. And that what is more likely to happen is children of related couples will have less genetic variance and thus a whole clan of them will succumb far more easily to disease and illness.
I think it has to do with their God that they descended from, where if people remembered their God that in turn the clan would've survived, however since the God was not remembered or found again, the clan and to make do with that themselves, turning themselves int something to be worshipped so they could be remembered.
If they could be remembered as a clan and not as much of an individual, they may be able to revive their God, and in turn stop having incest?
Unless it has to do with them not being remembered there is no reason for them to keep reproducing if they can maintain being remembered / worshipped.
I'm surprised the Aehri clan's children/offsprings didn't suffer some abnormalities about having an incestuous relationship. It is messed up but to think that they'll automatically die when they had a child with 'normal people' or people outside their clan is saddening. And it's even their way of committing suicide.
Is their clan cursed? They'll die if they will be forgotten and then when they die, no one will remember about them. :/