I am sorry to hear that your daughter is acting out. I have two girls who are two years apart in age. They are both over 18 now, but when they were preteens & teens living at home they were a handful!
The younger daughter was always saying I loved her older sister more, and that I always gave her more stuff. It wasn't true, but I think some kids just need more attention than others.
I think all you can do is show your daughter that you don't give her brother more, and then perhaps try to spend some more one-on-one "girl" time with her. She's probably starting to feel like she's growing up, but conflicted because she really just wants to play and be a kid. She sees her brother carrying on the same since he hasn't hit puberty yet.
But, my biggest tip for you is to give yourself a break. Your daughter is pushing your buttons, but you don't have to give in and you don't have to feel guilty. You're her mom, not her friend, and sometimes she is going to "hate" you. You know she really doesn't, but she needs to vent it out. Just let her do it, and be there to hug her when she calms back down.
Anyone else have a teen son starting high school this year? I've gone through two girls, but my son has been different from the girls in so many ways. He's also ADHD and his self esteem sometimes flounders.
I'd love to get some pointers from moms with sons who are or have been navigating the high school waters.