Cheating, abuse and lies. Should I tell her?

My dad just announced that he proposed to his girlfriend but I don't want the marriage to happen. They started dating in February after her husband died in December. I love the woman and her kids to death they're wonderful people who I care for. They aren't the problem, my dad is. 4 years ago my mom kicked my dad out because for the 3rd time in 27 years she found out he was cheating including the time he got a woman pregnant with my brother who he refused to admit exists. What was worse is he forced me to hide it from her and had the balls to introduce me to this woman who herself was married as well. My dad swears he's changed but I don't believe it. My whole childhood he put me down and slapped me around just for the hell of it. Now he tells his new girlfriend that he was the good parent and it was my mom who treated me like shit. He's even went as far to tell her I'm lying and making things up. When my parents divorced he claimed my mom stole $20k from his secret bank account, that we had lived in South Dakota for 5 years, he had his pilots license and a college degree. This man is a convicted felon, didn't finish high school and has worked under the table jobs for 30 years. He's made up such a world of fantasy that everyone loves him and believes he's the greatest man and father ever. I'm almost 24, married and trying to start a family of my own so I wonder if it's my business to tell her or if I should just let her marry him. I'm scared for her because he slapped me and my mom around so who says he won't do it to her. Also I don't believe she knows about my brother, my dads criminal history or really anything about him. I'm afraid if I tell her my dads side of the family will cut me out of their lives and tell everyone I'm the bad guy again for not covering for him.