My sister genuinely won't change
For years my sister (16) has been stealing, lying, breaking shit, stomping and yelling. At first we thought maybe it was just anger issues, that can be resolved. Maybe tantrums related to the ADHD. It's only gotten worse. My parents have yelled, hit, talked, ignored, I feel like they've done all they could. She herself has said "saying nicely isn't going to work either". Recently it's gotten to the point that she feels she has to have a snarky remark to anything and everything.
I no longer try to associate myself with her. I don't want anything to do with her and while my parents disagree, they don't argue.
She doesn't have video games or tv anymore. All rooms besides mine and bathroom have locks and keys because she'll just take shit. My parents room, the spare room, the bathroom closet, and we almost put a lock on the basement. My room is in the attic and has creaky stairs so lock isn't really needed.
Shes spoiled in the way that she lives in her own little delulu world and severely needs a reality check. Shes so used to this violent and loud house that she no longer cares about being yelled at or hit anymore.
She's brought CYS into the house several times about exaggerating or just straight up lying about mum hitting her and shit. They never do anything, usually we just explain the shit and they leave. They did bring in a therapist for a couple months but she wasn't super helpful. She implied that if my parents just "loved her more" and rewarded her, she would be all better. We did start to see a bit of improvement, but as soon as she got caught stealing again and got things taken away, she went right back to shit.
My parents have sat her down countless times to reason with her, and she's very aware of what she does. She says she wants to change but we never see any effort of her trying to change. She wants the same things I have, and she wants my parents to stop being so distrusting, but she won't do any of the work or take any of the time to build new relationships. My parents have thought about sending her to institutions but haven't gone through. She doesn't have friends, and is in the "special" class with drug addicted minors and shit, and isn't allowed on school computers due to looking up porn on them first chances she gets.
My parents don't know what to do. They don't want to kick her out, shes still their kid. They cants reward, cuz she'll just expect to still get away with shit, and they can't take away, cuz there's nothing else she has left. It's been years. Shes done shit like this since she was around 6. They cant baby her, and she doesn't act old enough to treat her her age.
Edit: I will add that every morning before school, all mum does is try to get her to get ready for school, and she tells at my mum obscenities. Tells her to fuck off, fuck this, she hates her, and she hasn't done this in a while, but has told my mother to kill herself