Well, I think if most of her family already know the truth, I would probably leave it. I would bet that her Mum does not want to think that her D was the one to bring about the end of her M upon herself through her infidelity.
Your W may not even have lied about it - but equally from your sitch you have said before that your W believed you may have been unfaithful even though you weren't. The thing is, I think it is difficult to tell your parents - I had an affair.
My guess is that her husband and S will call W's mother out on it, even if you do nothing. If you have already had that convo with W's brother, he'll know from your reaction that you are pretty upset about it.
I put out a bat call to Sandi on newcomers...maybe she'll stop by too. Don't do anything just yet while you are upset....
T 13 M 7 Me 48 H 46 SS 15 BD 7.14 PA D final 5.16 (H filed)
We receive & we lose, and must try to achieve gratitude & embrace with whole hearts whatever of life that remains after the losses - Dubus