I don't know how to help my sister

TW: mental illness, mentions of suicidal ideation

My sister (26) is severely mentally ill, with depression, anxiety, OCD, & ADHD. And, in my educated opinion and also in my partners' psychology-degree-informed opinion, she has undiagnosed narcissistic personality disorder and autism. She also has Tourette's and some weight-gain issues, potentially PCOS-related. She has no in-person friendships aside from myself and my partner (she calls us her best friends). She spends most of her free time online, where she has some online friends who she texts and has calls with every so often. She is extremely selfish and shows little understanding for or ability to compromise with others. She totally lacks any sense of self-awareness. She is the most insecure person I know and consistently outsources her emotional regulation to others. She asks for reassurance in relation to her body, her worth as a person, her character, that her loved ones love her, etc. She can be manipulative, purposeful or not, to get this reassurance at times. She sometimes makes worrisome, attention-grabbing posts hinting at suicidal ideation. When she texts, she demands a response to each message. These all compounded have caused her lots of issues in her friendships, where she oversteps boundaries and/or asks too much of a person and they argue for days and eventually have a "friendship breakup." She has bad interpersonal skills irl (cuts people off, talks for way too long, doesn't understand the flow of conversation, talks only about herself) and bad motor skills. She is time-blind and bad with transitions from one place to another. She has hyper-fixations and parasocial obsessions with music artists. (some of which have caused problems on twitter in the past). She spends a lot of her money on merch, toys, useless things, secret stashes of cookies, chocolate milk, sweets and sodas. My mom cooks and cleans for her, drives her around (she doesn't/can't drive), plans her appointments and basically is her full-time care provider. My sister has a horrible sleep schedule, sleeping late into the day and has a bad diet. She is addicted to sugar, which recently made her become pre-diabetes. All of the advice her loved ones give her is not internalized or accepted at all. She refutes or deflects everything we say to her. ie: "maybe you should try to exercise and go outside more" is met with "I already do that, I bike a lot." or "you should cut down on how much soda and sweets you have" is rebutted with "yea i have been, I drink low-sugar and barely eat those." even though that is not the truth.

My sister went to a community college and got an environmental degree, but didn't really make any lasting friendships. It's been really hard for her to find a job because she can't drive, employers are ableist and because she's picky about the work she does (refusing to work a grocery clerk or other basic minimum wage job). Recently she did finally landed a part-time role at a zoo, which she's been aiming for over the last 2 years. Which is hopeful! Since she's graduated, though, she has gotten much worse with all the issues above, and gained a dramatic amount of weight. She thinks she is doing things to better herself but she really isn't. And in some areas that she needs to better herself she doesn't even understand that she does. Introducing those ideas to her cause her to have hours or days-long meltdowns, with us having to repeat advice in circles and provide her with reassurance that she's still a good person, etc.

My mom, my partner and I are exhausted from this behavior and don't really know what to do to help her. She is 26 and doesn't seem able to take care of herself at all. My mom and I talk about getting her an apartment nearby and helping her with groceries and other bills, but worry about her being able to keep a job and pay rent. She also has no cooking & cleaning / general adulting skills. She seems to listen to specialists and doctors just a bit more than her family, so I've considered trying to find a therapist to see with her. That route would involve a nightmarish insurance processes (USA 🦅🔥) and finding a specialist, and me to devote a lot of my time and energy (I'm struggling with mental illness myself!). I don't even know what kind of therapist/psychologist to even see because her problems are so deeply rooted and all-encompassing. And then of course we've thought about placing her in some sort of group home, which would have its own set of life quality issues and financial stresses.

I'm just so overwhelmed and I don't know what to do. I've moved out of the house but I can't imagine what it's like for my mom currently. It was really hard living with my sister and greatly helped my mental state to move out. My mom deserves to have her own life and her free time.

If anyone has any ideas or thoughts or advice, please let me know.