If your mom is planning to marry someone whom you don't approve of, the only thing you can do is let her know how you feel. You can let her know it isn't good to fight all the time with someone and that you think the relationship may not be the healthiest. What she chooses to do though is out of your hands and you shouldn't take it personally.
Maybe you should start by asking her mom how she feels. Make sure she knows that you think she is awesome and that you're going to love her no matter what. Sometimes, she just needs to know people are there for her. Let her know that you want her to be happy and tell her you're worried she won't be happy if she stays with the person she is with now.
There is nothing you can do to prevent her from marrying someone you dont approve of. What you can do is realize that verbal and physical abuse is wrong, and to never get used to it. Everytime you hear or see some type of abuse recognize it as being wrong. If he ever hits you or starts to down you tell your mom, then if she fails to act tell a trusted, trusted family member whom you are sure will remove you from the situation. The worst thing you can do is "get used to it" because whether you realize it or not it will effect your future and your relationships in general.