Your anger sounds justified to me. It is wonderful when parents and adult children remain close; however, it sounds like your husband and MIL's relationship definitely leans toward unhealthy codependency. It is never okay to use one's child, even an adult child, as a therapist or surrogate spouse.

In my experience, your husband will need to learn to set better boundaries with his mother if this situation is to improve. Can the two of you seek therapy together to work on this?