It wouldn't be unfair of you to ask him to cut that cord. When a new family is made the parents must step off. My MIL lived her whole marriage in the same situation with her MIL, it's really awful. Honestly, I don't believe this can be solved without involving some... harshness.

I, personally, wouldn't want to go to family therapy with her unless I considered her my family or she lived with me. It's up to your husband to set boundaries, so you should tell him what's happening and ask him if he want to see a counselor to help him deal with it... and help his mother finding one too.