Sharing Our Innermost Thoughts

Hi everyone, my step sister has gone off the rails this year with hurtful bad behavior torwards everyone in her family and she’s hurt me deeply. I was the first one to find out she snuck out and didn’t know what to do and told my mom but was up all night worried about her and was wrecked the next day. Everytime I’ve gone out of my way for her and I think we’re good, giving her advice or just doing little kind things a few days to now a few minutes later she’ll turn around and just be mean passive aggressively mainly when I’m minding my own business or of she’s just in a bad mood and even if I sympathize with her and try to help she’ll lash out yelling at me or just completely act like I don’t exist. It really hurts when you give from a genuine place with no other feelings and someone just stabs you in the back. Recently her mom told her no to prom bc she’s a junior. She goes to her mom’s on the weekends. But she twisted it and called her dad throwing a tantrum and even getting grandma and his sister involved saying she wouldn’t let her go over there and it’s his fault when her mom was the one who said no. I talked to her about it and about doing what’s right and having integrity being humble not trying to force your way and being honest and she replied saying I didn’t know anything and that she wasn’t trying to be mean I told her okay I wasn’t trying to me be mean either I was just remembered my days in highschool and wanted to give her a warning but if she didn’t have to listen if she didn’t want to. I meant that there was no other motive. She really didn’t have to if she didn’t like it. She was fine when I said it earlier, I just added the details about highschool as to why I said what I had. But she ended up lashing out right as I thought we were at the same place of not being mean and just communicating and I told her I loved her. She insulted me and tried to manipulate me by saying that I pretend to have her back but I really don’t. I asked my mom what to do she said to just not bother or give her any attention. Any advice for the situation on how to handel it? I’ve had nightmares and have been afraid to be around her bc of how she acts torwards me.