Adoption Relinquishment??

Hey I actually don’t know how to explain the situation in a title, but I want to start off my saying my little sister is the sweetest, smartest, and most kind Individual you will probably ever meet. She is so bright and lights up a room in an instant, but she lives in a foster home with genuinely evil people.
They constantly isolate her, won’t let her hang out with friends, she does all her school online so she has no social life, and rarely let her outside. They constantly put her down by making her feel unimportant, and telling her things such as she’s dressing like a whore, letting her cat roam outside which he isn’t allowed to, going through her room anytime they feel like, taking her things from her and hiding them, such as clothes she got from SHEIN, her phone, or clothes they don’t like. My little sister is almost 15, and she has no privacy. They make her clean their entire house and do nothing themselves. They don’t listen to her when she asks for a therapist, and constantly belittle her feelings. She is extremely depressed and has started to self harm, which they know about and choose to do nothing.

Last summer they let her stay with me and her other sister for the entire summer. Her scars started to heal, she had a social life, she helped with the chores, and never hid in her room. She was getting much happier and they were telling us it’s a possibility she could stay living with us, and all the sudden decided never mind, she’s not allowed to, and rarely let us see her anymore. She has told them she hates them and that she hates living with them and doesn’t want to be there anymore, and they just made it out to be victims telling her things like we do everything for you and you’re so ungrateful, this is why we don’t let you do anything, you’re such a bad daughter and so on.

Today she told me she’s going to run away, and I don’t know what to do. I’d love if she came to live with us, but I can’t afford a custody battle. She told me she’d rather live anywhere else, even in a foster home or a group home than with them, so I’m here to ask what do I do?

We have recorded conversation of her foster parents telling us the only reason she couldn’t live with us was because she was too dependent on me and her sister and that they’re lonely at their house, which to me do not sound like valid reasons. They did not say they missed her, they did not say they loved her, but that her happiness was too dependent on us, and that the house was lonely.
I’m tired of sitting around feeling sorry for her, when I can be doing something to help her, I just don’t want to make it worse for her, so where do I even start?
Location: Arizona