It's great that you want to help your sister. I think all you can do is to continue to educate yourself about autism and be there for her.

For example, while, of course, everyone here has the right to identify however they want, high and low functioning are outdated lables and even considered harmful by some. I'd recommend reading up on that. Another thing that I recommend is looking into PDAPDA or pathological demand avoidance. Understanding this might help you understand her reactions better.

If she rejects her diagnosis, let her. Forcing it on her won't help her. You can try to change your language around it. Maybe instead of mentioning "autism" outright, only talk about sensory issues, for example, and try to accommodate needs one at the time. (Sensory issues aren't exclusive to autism.)

About finding community, encourage her special interests. She's much more likely to find friends in an environment she enjoys and identifies with. But even if she doesn't, try to tell her that it's okay. That she's okay and that there is nothing wrong with her. That even if she never feels better, she is loved. You are her friend, and you love her, and you don't judge her. And you won't force her to do anything she doesn't want.

Only she can choose to go to therapy or seek out support groups for herself. It won't work any other way.

Wishing you both the best of luck.