My stepfather's verbal abuse triggered body dysmorphic disorder, eating disorders, low self-esteem, and essentially ruined my school years and interest in playing the musical instruments I loved playing. There were times I was self harming and suicidal because I didn't feel that I was worth the life given to me and I believe that it was a direct result of his verbal abuse.
You absolutely have to make him stop. My mother praised me and supported me and it could not erase his hurtful words. And I agree with the pp who said that she is going to grow up to resent you. I cannot forgive my mother for not standing up for me. How could she let him make me think I was worth nothing!?
Have you ever tried telling her right then that he is wrong about her? Interjecting immediately in her presence that he is wrong about her and that he is way out of line to say the things he says? Telling him in front of her that his behavior is not okay?