I agree with the others that your daughter does not need to try and see her. I think the reason you are trying to accommodate that is because your STBX has conditioned you to. Don't condition this brilliant young lady to do the same.
My mother is like your MIL. I have a brother 8 years older than I am, and an identical twin. My mother was always totally obsessed with my brother. He was constantly failing in school and my sister and I got straight As, but the sun rose and set on him. He got whatever he wanted, while we got ignored. The result is that I kept working harder and harder to win her approval, which would never come. It set me up to do the same in relationships. I didn't see red flags in men. I saw challenges. I'd get someone to really love me if it killed me. It darn near did.
I reached my "I'm done" moment when it came to how she treated my DD. When my DD got married, my mother was all excited and couldn't wait to come to the wedding. Right up until I told her my brother wasn't invited. I hadn't seen him in years. And I made it clear that my sister would be there, and she had been sexually abused repeatedly by my brother for two years between the age of 8 and 10, which our mother is well aware of. My DD was also a victim of a family member on her dad's side of the family so, I had no intention of having my pedophile brother at her wedding. So grandma didn't come to her grand-daughter's wedding. Golden boy was more important.
That set me free. I had one conversation in which my mother insisted my sister made it all up, in spite of the fact that he openly discussed it with me years ago, explaining why he targeted her and not me. She insisted on calling my sister and I liars. I didn't speak to her again for two years, no matter how many sobbing voice mail messages she left.
It took years but once I stood up for myself there was no looking back. My mother is now 90 years old, and struggling health wise. She lives about 3 hours away from me. I went to see her once in the last 10 years. I only speak to her once a week for a half an hour. And I don't feel bad about it one bit. There are people who are not worthy of our continued effort, family or no family. She is one of those people.
Good for you for telling MIL off. I'm not surprised the other family member threatened you. When someone finally speaks the truth, they can't stand it. But you don't need to stand for it any longer. The sooner you have the whole lot of them in the rear view mirror, the better.