Is it Wrong of me to Not Let Grandma Alone with Baby?

My boyfriend and I have been together for 3 years. I didn’t meet his mother until last year when she moved back home. We didn’t want anything to do with her, but decided to put things aside for his grandparents which are the ones who raised him. I don’t know why she didn’t get arrested for this or exactly what happened or who did it or why she didn’t do anything about it, but when my boyfriend was a baby is grandfather went over to his mother’s apartment to see him. When he got there he could hear my boyfriend screaming and crying in the bedroom. He asked what was going on and her and her boyfriend at the time said my boyfriend was fine and not to worry about it. The way he was screaming and crying his grandfather knew he wasn’t fine. He told them to let him in the bedroom, but they wouldn’t. He had to kick the door in because it was locked shut. My boyfriend was laying on the bed covered in blood and letting out these blood curdling screams. His grandfather called 911 and he had to be life flighted to the hospital. He had multiple broken ribs and had to have multiple surgeries. He died, but they were able to revive him. The only people there for him were his grandparents. Not long after this had happened his mother moved to Puerto Rico and has been there since May of last year. She had no contact with anyone. My boyfriend’s grandparent’s phone number has never changed. Her sister even sent her a stationary kit so she could at least write them, but she never called or anything. Then out of no where, 26 years later, she’s moving back and with her parent’s, so there was no way to avoid her. For a while we did avoid her, but avoiding her meant that we wouldn’t be able to see his grandparent’s and we could tell it was making them really upset. We put things aside just to be able to make them happy. My boyfriend started talking to his mother more and more. Out of no where one day, when it was just her and my boyfriend, she thought it be the perfect time to talk about what happened all those years ago when no one else could hear what she had to say. She said that it wasn’t her that did that to him, that she wasn’t even there when it happend. She said it was her sister that did it, which doesn’t even make sense. I know I wasn’t there to know what exactly happened and he was too young to even remember, but I think I take his grandparent’s word over the one who ran to another country after it happened and had no contact. Now that her parent’s are getting older she magically returns, but too bad for her they already had their will written years prior. She had 2 sons while she lived in Puerto Rico. They both moved here, but a state over with their father. The youngest decided to move with her a couple months ago and now we barely see or hear from her anymore. She never cared in the first place. We always had a feeling she has nothing but bad intentions for coming back. I don’t trust her.