Stop hitting her

Tonight I sit and wonder about my mother. I am 26 years old and feel like I still have to protect her.
My stepfather used to beat her up all the time when I was younger. He hasn't hit her in 10 years and then one night 2 weeks ago I came home to find the cops there arressting him. Yet she takes him back again.
I moved away 2 days ago and i am so worried. They are moving in December and my sister and neice are going with them. My sis was too young to remember what I remember. And its her real father so its different for the both of us.I am just concerned and I dont want my neice growing up in the same environment I did.
Well I have to go on living my life.I have alot of work I need to do on myself. But I just wanted to express this feeling I am feeling right now. Anger and confusion on why would she take him back,again,after all these years!