Physical laws cannot change by definition. If they did, they wouldn't be laws. But if the laws as we know as today, started to change, then we would realize that actually they are not laws, but only apparent laws subject to change according to real unmutable laws.

Now if the physical laws as we know, were allowed to change in a random non deterministic way, then the overall result is that there is no physical laws at all, because a law that is subject to change at any moment, cannot predict anything with certainty.

Now, in a universe in which there is no law at all, is highly improbable that stars and planets arise, let alone life.

Fucking your sister is probably going to destabilize family dynamics, no? — darthbarracuda

I think that could be true, but merely for the fact that incest is prohibited. Fucking your sister will probably lead to gain others family members rejection, and thus destabilize the family. Even without others family members noticing it, just for the incestuous couple to know they are doing something which is prohibited would create in them some disturbance at least.

The key statement to decide is whether incest would cause family destabilization in the context of a society which does not condemn it.

That said, I think your argument is the main implicit argument behind the law besides the weaker genetic argument.

I think there is a collective fear to incest and that fear resides in the idea that incest leads to desarticulation or perversion of the family as an institution, which in turn is, for the eyes of the conservative mind the base of society.

So in that conservative collective imaginary, incest, in the case of spreading out, could destabilize a society.