The thought of it is quite odd but then they just may be more stable. after all it takes a village.
Our western way of thinking may be the issue. But then how do you get a kamikaze pilot to fly his plane into an American battleship? Some activities are questionable. Others are just weird.
It is not more stable. It was reserved for god-kings and it is not healthy.
If you look at any history you find that any family that practices incest dies out in four generations.