Step parenting

Mood today dealing with my ten year old whose mother hates me and is constantly brainwashing her ! Like why ? Why do ppl do that. It’s a constant fight and battle. Her mother never is around and always want to go out and party so we have her. But she is so disrespectful and constantly fights , brushing her teeth, combing her hair , showering just basic stuff. Is this normal ?? She tries to play games with me to get her dad and I fighting which I’m not gonna fall for. I’m sick of it.! I have a 18 month and 26 weeks pregnant. I miss the sweet little girl she was ! I don’t understand who this new person she is but it’s honestly like dealing with her mother in small form. I’ve been married to my husband for 6 years now. So I’ve been in her life since she turned 5 but she’s changed so much. She’s such a different child. Idk how to stay sane.