What a horrible, horrible situation you and your family are in. There has been so much good advice here on this thread, and I have nothing really to add. Except that I agree with those who have posted about your MIL, she may seem like a wonderful person, but she has chosen to stick with her husband. And that right there is enough reason to not trust her anymore.
Massive hugs, I really hope that you guys get some counselling and can move on. What an absolute mess.