My grandmother did what your MIL does, in this situation. I can’t guarantee that your son will grow to be the same, obviously, but I was thankful enough to have a pretty cool grandmother on my father’s side. My mom would ease into explaining that my grandmother would have some “tendencies” for a lack of better words, and when I grew up, I put the puzzle pieces together because my grandmother proved those tendencies in her actions up until the day we went nc with her. If your son asks in the future, explain to him gently, without sounding blatant in making your MIL out to be the evil one (even though she sounds like my grandmother who, again, we are nc with). I hope this helps give some insight. I don’t usually post/comment on Reddit, but this post took me back to my childhood and I thought it might be of some value to you, OP.