your mil sounds a lot like mine. just like my ah's addiction belongs to him so does his mother's co dependant ways belong to her. i do understand how you feel. i have felt as if i was in competition with my mil from day 1. i also think maybe their relationship is kind of based on guilt from his childhood and that in itself along with his addiction has brought much added stress to our marriage.
i had to separate myself from the both of them for my own peace and sanity. for yrs. i had a lot of anger towards her. i guess i wanted to be in control of their actions, too, i guess i thought i knew what was best for everyone.
someone told me once that i couldn't change other people but i can change my attitude toward other people, and this helped me to past my anger.
i'm sorry you are going through all of this and i'm glad your ah is going to rehab. even though thats a good thing, you still have to do what is best for you. i pray that he makes it this time and i pray that he continues focus on taking care of you.