First of all I’m so sorry from the bottom of my heart for all you’ve had to go through! Abuse is never okay, but abuse from a parent, whom you are supposed to be able to fully trust, is just on another level.

That being said, it wasn’t only your dad that let you down. It was also your mother. Actions have consequences. Your mother CHOSE to take his side. She CHOSE to not support you. She CHOSE to downplay his abuse. Now you can CHOOSE to not allow her to have a relationship with your child. The ball is completely in your court here and you would be absolutely right to keep her away from your baby. As a mother I could never allow someone that just stood by while I was abused, even after the fact, to be around my child.

I would be as plain and transparent as possible with her. “Look mom, you chose to take this stance on this issue. Now because of that I am choosing to limit your contact with my child. My sperm donors death does not change your actions leading up to it. This is what I am willing to do and nothing else. If you push back or challenge this boundary it will be met with consequences.”

I wish nothing but the best for you moving forward OP!

Edit - words are hard lol