As an adult, you have choices. A good book is "Boundaries" by Townsend and Cloud. I highly recommend it! you do not "have" to do anything someone tells you to do.

Don't allow your parents to "guilt" you into anything or manipulate you. That stuff is emotional blackmail. Don't discuss your sister with them, or respond to anything you aren't comfortable with....this is where good boundaries come in. Knowing when to keep toxic people out and or/let certain people in.