Another thought, if your SMIL has a MSW degree and training in counseling, perhaps she'll come to you to ask questions. Even if she doesn't, she knows there are 2 sides to every story. And I agree that parents know their kids. If they had the same spew thrown at them 20 years ago, they know...
I don't think she will come to me. She is very pragmatic. She does not want to alienate my wife.
And you are correct. As I have said, she is smart enough to know the score.