The most you could do is ask her if she'd be willing to give therapy a try. Don't tell her she needs it necessarily (speaking from experience. People told me this all the time and it made me more resistant to the idea.) but ask her to give it a try. Maybe specifically ask if she could try to stick it out for a month before ruling it out as an option.
Or maybe, since you share the same diagnosis, you could ask her to try family therapy with you? You two could have sessions together and maybe it will lead to the Therapist asking if she could have some individual time with each of you. That may be a good gateway for her to get some help.