To novelty, avirah:
You’re assuming the entire reason for AMH is that baalei chaim. Aside from being a midah raah, it could be it’s only derabanan.
From the pashtus of the pasuk, it seems there is a much deeper reason of damo benafsho.
Additionally, (what I had in mind initially) there are many ways to do AMH without causing pain to the animal. But it would still be the exact same isur.
For example, if the animal was out cold. Or if it was one of those animals that can regenerate a limb, so eating that limb would be irrelevant to the animal. I can probably think of other examples.
Regarding sisters, it’s not the mainstream opinion that goyim can marry them. The gemara says a ger cannot marry his sister even after they are both megayer because we don’t want him to enter from chumra to kula (meaning he has a chumra beforehand.) I saw somewhere but don’t remember where, that he can marry his sister from his father but not from his mother.
My point was that it is possible they do have chukim. Not saying for sure, but it seems that way. How would it impact their bechira if we say they do have chukim?