Oh sweeite! I feel your pain. I was completely blind to my husbands daily crack habit (along with ALL his friends). I hadn't filed divorce papers, but I had written a letter to him one night during his disappearance act stating that I didn't feel the kids and I were part of his life, and that we were keeping him from his true desires. This was written before I knew of his addiction to crack.
You are complete right in your feelings. So don't let anyone tell you otherwise.
In the meantime, I suggest turning to your High Power for answers and guidance. Since I have, I've been reminded of my vows and that our (mine and my AH) HP forgives us of our sins. What our AH's have is an addiction, which is an illness. Yes, they've chosen to use, but they need help to learn to make better choices. I'm lucky in that my AH decided to get help. I know he'll probably relapse, but we'll cross that bridge when we get to it. It is taking it one day at a time.
If you decide that you do want your AH to be a part of your life, you have to set your own boundaries. My first was telling AH that if he couldn't go through with the treatment program, he couldn't come back home. This was after he told me that he didn't think he was in the right place for himself -- thought he could do it with out-patient treatment (HA!).
I've not only spoken with our Rector, but I've also found myself a good counselor to help me through the 12 steps for co-depency (sp). One of the things that my counselor has taught me is that while I'm not addicted to anything, I am a victim of the addiction. And that before I can get past the anger and *allow* my AH to begin to earn back my trust, I have to work through being a victim of addiction.
Don't let your MIL talk you into anything that is outside of your boundries.
****{HUGS}}}