The only route I could suggest you try is to go to a lawyer, present the unhealthy ways your sister has been caring for your parents, and see if they will take your case.

I know from experience, your sister is unlikely to change unless something catastrophic and enlightening happens to her, and it sounds as if your parents are in no shape to either change things legally themselves nor "escape" in a legal manner.

My heart breaks for you and your parents.
*hugs*