It's all well and good to love one's mother, most of us do. But once we become adults, our spouses come first. It's your fiancee's duty to cut off her mother if she's causing drama.
I would talk to the fiancee about limiting contact or if she refuses, at least let her know that you will not be around your MIL after the wedding. She should respect that.
Like Dr. Phil says, you do not reward bad behavior. Continuing to allow her in your life is sending the message that what she does, cause drama, is acceptable. It's not.
I wouldn't lash out because that's feeding into it. Negative people want to spread their misery. Aggressive people want to provoke a reaction. It makes them relevant.
High hopes faint on a warm hearth stone; she travels the fastest who travels alone.