My mother is mistreating my step sister. How could I help her?

Anonymous
My mom married a man 7 years ago, and he used to have a daughter of my age. We are now both 18, and we live together. My mom always hated her and encouraged me to hate her too. For example she would tell me, as a kid that i should do everything I can to make her dad loving me more, or even when I have done something bad she would throw the shade on my step sister. I admit that I used to be a real bitch at 11 because I was imitating my mother by bullying her. So to please my mom, I was putting her down. I stoped to bully her when I saw my mother beating her to death in front of me and I felt bad for her for the first time. now I feel truly bad, because my mom hate her so much that she wants me to start a rumor about her in school, that she’s a whore so that her father will hate her, but I don’t want to. I feel that my mom is the evil stepmother of Cinderella and she is forcing me to be the evil step sister. My step sister is really kind and sweet but my mom is always bullying her and I feel so bad because I love my mother and she love me. But when I ask her to leave my step sister alone she would tell me that I’m a doormat, and that it’s none of my business ! What can I do to help my step sister without offending my mother?
My mother is mistreating my step sister. How could I help her?
My mother is mistreating my step sister. How could I help her?
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