Thank you for writing!
I remember helping you on this matter several years ago. I'm sorry things have not improved.
If your family feel that your sister is a danger to herself or others, you can talk to medical professionals and have her "sanctioned" which means forced into medical care for a certain amount of time. These sanctions are not easy to get but are certainly possible.
However, it sounds like your sister's concerns are very bothersome to you, rather than dangerous. If you live with her, consider moving out. Adults siblings are not necessarily meant to live with each other. Getting some privacy and space will help.
If your sister is living with her parents, they can encourage her to seek medical help but the onus is really on her to see help. She cannot be forced to therapy or forced on medication if she doesn't want help. Another alternative would be for your parents to remove her from the family home so she is forced to seek help although that may backfire and she may end up homeless. You are limited in what you can do. Since you are a grown woman yourself, find a place of your own that you can afford so you can have some peace of mind. You can always visit home often and help out if needed.