How do I help my mom??

WARNING….mentions SEVERE domestic violence….

I need some advice. I’m at a loss on what to do. My mom is just under 50 years old. She has been in an abusive relationship for a long as I can remember. My mom as been with my stepdad since I was 5 (31 now). When I was 12, I started to notice how abusive he was towards her. He’s always been an angry drunk. When I was 13, I walked in on him choking my mom. A year later he cut her on her back on purpose. I believe he served jail time for that. He’s also cheated on her. I thought at some point she would leave him, but she didn’t. They didn’t get married until 2014. Thankfully, I moved out when I was 15 into my grandparent’s house and never went back. I was hopeful things would get better, but my 3 younger sisters would often call me and say that they were fighting. My stepdad is the father to my 3 younger sisters.

As time went on after they got married, they would have bad arguments, but my mom never left. For a while, it did seem like things were finally getting better, until this past June. My stepdad got really drunk and assaulted my mom and two of my sisters who are 24 and 19. One of my sisters ended up with a bloody nose. My stepdad was arrested and charged with assault on each of them. They got a protective order. My mom said that she was going to start the divorce process. I thought to myself that this is finally it. She’s going to finally leave him. My sisters and I were so relieved.

WELL, my sisters just informed that they think our mom is “sneaking around” with my stepdad. My sisters have noticed that they have been in contact, and think that they have been meeting up. This seriously pisses my sisters and me off! I can’t believe It! I don’t understand this at all. My mom has a successful career. My youngest sister will be 18 this year, so it’s not she needs my stepdad. At this point I’m convinced there is something seriously wrong with my mom.

I know it’s really not my place, but this is my daughter’s grandmother, and I refuse to have to explain to my daughter that her grandmother was killed due to domestic violence. I know that sounds awful, but at this point, I’m scared it’s going to happen if she takes him back again.

What do I do or say?? How do I approach her about this? A part of me wants to tell the police or whoever that I suspect the protective order is being violated, but idk if that will do anything.

Any advice is helpful.