The hardest thing to accept (or at least for me) is that I was powerless and that I could say my opinion but if the person doesn't want to hear it then I have to step back and accept that it is not my job to run someone else's life- that they have to make their own decisions just as I make my own decisions. I know your first reaction is to do something, stop this from happening, get her to open her eyes and remember how bad it was and how most likely nothing has changed and that she is reintroducing a man who abused her and her son into her life...but you can't stop it from happening.
What you can control is how much you allow the situation to affect your life. Even though you won't be the person living with him, it could still take over your life and can affect your mental and physical health. It might be a good idea to reach out to people who you know will be there for you and set up a healthy support group you can lean on. It might also be a good idea to set up some boundaries and decide to what extent you will allow something you can not change to run your life. I am one of those people who recommend counseling for any and all situations but it could really help to have an unbiased opinion and that can let you know when you are obsessing over the situation or letting it affect your health.
I will be praying for your family and for your mother. I am so sorry that you are faced with making the decision of do you let it consume you or do you step back and have to accept that you can't control it. I know it is hard and easier to say then do, but it might be a good idea to think about what you are going to say to your mom before you say it and ask yourself am I saying something that is trying to control my mother's life or am I saying something that is healthy and being there for your mom when she needs someone to lean on in this stressful situation.
HUGS