Oh man. This is almost impossible to deal with and the situation of nightmares. I have some experience with this.
The therapist has more power, control, and credibility in court than your entire family put together. The state will always side with the therapist and the therapist will always side with the patient / against the patient's family. Your family is in a 2 vs 1 situation over your sister (3 vs 1 if you count her).
I know that you know "a danger to herself and others" could realistically mean she's a drama queen and threw a temper tantrum when her family criticized some of her life choices on a holiday, but if she brings this up in therapy, then the official notes (which the courts will go by) will label it as abuse / child abuse (I know). So, do not confront the therapist or support group yourself. They might have actually already blacklisted you from visiting the group on the grounds that you or your family are "abusers" of some sort who have contributed to her condition (I know), so they can always block the family from seeing the patient. Some outpatient centers even have this policy by default for anyone above 18 - no contact with the institution is allowed form the family. It's a black box only she is allowed to enter, and you can never find out what's going on in that box. You can only interact with your sister's therapists through a third party in this situation. If you yourself try to confront someone or sort it out the way a sensible human would resolve it, it will make it worse. If you've never dealt with something like this before, it will likely blow you away learning how much power these institutions have over your family members.
There is a legal way to grab full control of your sister. Idk where you live, but on the east coast in the US, I had a friend whose family did that to his sister. In the event of her being declared mentally unstable, the family is allowed to take full control of her as proxies (her finances, her bill payments, where she lives, etc.). Even then, they were still having big problems for years setting this up because HIPPA laws caused a bureaucratic road block.
If you want to pull her out of the program, the safest way to do it is see if this program violated meaningless bureaucratic regulations (Did they hand her a copy of her patient's rights? Did she sign a consent form where the therapist can prove she was in a sound state of mind? Is the building built to fire code? If there's food at the support group, has it been certified by a health inspector? When was the last fire alarm inspection? etc.). I know that you know these rules are meaningless, but that's how this game is played. It's harder for a family to get a grip on an out of control child in the mental health system than it is for a man to get full custody of a kid in the divorce courts. In this situation, the state sees the therapist as more important than the parents because they have a degree or certification to handle her and you don't (I know).
Sometimes, getting her out from therapy requires the consent of the therapist themselves. Her family stands no grounds on removing her from therapy altogether because, legally speaking, the patient has told her story to the therapist and not to her family (I know) and the family cannot be deamed as experts in handling it while the therapist can because the therapist has sat through training programs. If you take these same training programs yourself, it doesn't count as having authority because you don't know what the issues are since HIPPA shields the patient and only the therpaist is recognized by the state to know what's going on with your sister's life (I know).
The system is designed for a slim minority of cases, not the majority of cases. eg. Your sister is being immature and disobedient. Your family trying to pull your sister away from this "help" and straighten her out themselves will be treated, procedurally, as exactly the same as a woman in full psychosis with her rapist coming to stalk her while she's trying to escape him (I know). It's the same procedure. There's no levels of escalation to it, and it operates on a "what if X, then we're liable for Y" logic.
If there's a way you can legally become the therpaist here, try it eg. say she's on hard drugs and you need to detox her at your parent's house without her phone (no drug dealers) if her therapist says drug addiction isn't in the notes, accuse the therapist of being legally unreliable to care for her because they weren't aware of the problem (I know).
The situation you're in is total BS. For what it's worth, as someone who has had to deal with this problem myself, I will honestly and directly tell you that lying and being sneaky are on the table and often necessary to deal with the mental health industry. eg. They likely won't let you pull her from the support group (unless she's under your parents' insurance somehow) without transferring her to another therapist (ie no therapy is not an option, only different therapy), then just set up a bogus appointment with someone else who isn't covered by her insurance and once she's out, don't let her go back in. If she is under her parents' insurance though, then your parents probably could force her out of the program either by directly cancelling it or declining to continue payments and make your sister pay out of pocket.
The therapist, support group, and social media circle are all there to amplify her and re-enforce whatever choices she makes. The therapist and support group are usually not allowed to make any judgements unless physical harm could be done. If she wants an OnlyFans, they HAVE TO cheer her on, otherwise that puts them at liability for causing more mental trauma on her (I know).
I could share more insight, but I think at this point I have to write some kind of conclusion. I'm really sorry to have to say this, but I think you might have lost your sister. And it sucks to be in this situation. Multiple systems have been rigged to let this situation happen and to leave you and your family powerless.
My best advice would be to either 1) cut her out of the program through insurance if she's under your family's and isolate her from her friends if she's early into the problem, or 2) step back and let her make the mistakes if she's already deep in the mental health system and regularly spends time with this bad crowd of friends. It might take years, but if she was raised right, there's a chance she could come around on her own and avoid doing something stupid until then. If she's too hot to deal with, I recommend 2. If she's confused and insecure, I recommend 1 or something else.
If you went with 2 and she was raised well enough to learn when she was clearly out of line, then you can expect a call from her in her late 20s or early 30s giving a big apology after multiple years of no contact.
