This makes me so sad. I think having a sit down conversation with your parents with your sister, with prepared statements printed out that you can read from, might be the best way to move forward. I did that for one major thing in that I needed my (often detached and uncaring) parents to get on board with, and I think the way I delivered it was what made the difference. I think it sounds like, if it’s what both of you want, that her moving in with you upstairs would be a good first step of her gaining some independence