I'm so worried for my niece

So a little backstory. My step sister has three kids two of them were taken away from her and they live with their biological grandmother. So then she gets with this guy and has a kid with him (my youngest niece) he's a drug addict and has been to rehab twice and now he's had a stable job and is working really hard to improve his life for him and his daughter which he hardly gets to see now because he and my step sister broke up. Well now she is seeing this guy for only two weeks. This guy is a security guard at a porn house. She left my niece alone with this guy already. I'm so scared he is going to molest my baby girl. I want to tell her dad but my mom is telling me not to tell him about it because she may find out who told who about her new relationship and never let my mom see my niece. I really would like to adopt my niece but my husband and I love several states away. I still don't understand why she still has custody of my niece. I want her out of that house and I'm scared for her safety. I was molested when I was around her age and I remember it so I know if it happens to her it will seriously screw her up.