Has anyone been through something similar?

My toxic half sister and my sons father who is also toxic but was only toxic towards me not our son have now decided to become a thing. My sister lost all 3 of her children to the state. She’s not mentally stable, she neglected her children, and my nephew needed trauma therapy as a toddler/child because of her. My sons father seems to be allowing her as a mother figure to my son, which I’m absolutely concerned about😩. I don’t want my son hurt mentally or physically because of her. She never cared about her own children’s best interest and now she has my son in her reach. She’s crazy she’s always been obsessed with me, and wanting my life. So now she’s trying to do that by literally living my old life. I could care less that my sons father moved on as I already have and I’m beyond happy! I just was hoping my sons father would have found someone good and someone who took my son in as their own too. Now I’m stuck in this situation and I’m stressing! Am I able to do anything legally? Has anyone been through something similar?

I read that ultimately I can’t have a say since their together on the days he has him but if I have proof she’s mentally unfit for a child’s life then I can be granted custody or he will have to do supervised visits.

I truly just want the best for my little boy but I know her personally and she’s very unfit..😩