need help with my sis

I have an 18 yo sister who was diagnosed diagnosed with autism and anxiety in the past year, at the moment she is taking an anxiety med. Thing is, I don't know what to do to help her, as she always "jokes" with killing herself. I believe she has a strong support system, our parents support her and she has a little group of friends who she trust a lot, she also has me and our oldest sister, who also supports her in every way possible. She dislikes the idea of psychologists because when she was 16 she went and was treated like a kid, and I don't know what else to do. How do I talk to her about this, how do I even get to stop her from having those thoughs..?