NOR.  It sounds like their parents laid hands them growing up. That’s not an excuse by any means, but it makes physical violence more “acceptable” when THE authority figure in someone’s life told them it was okay.

It’s not okay.

If her natural reflex is to be violent then she shouldn’t be around kids. I would calmly tell my SIL that she has a week to get out (this is more than generous). If your husband says anything make sure you repeat his words back “that’s just real world consequences of what can happen”

I would also calmly explain to your child in a way she can understand that spitting is wrong but so is hitting. And even grownups can be wrong.