Have you discussed the issue with the parents before? I'm sorry if you already said you did I may have missed it lol.

If you have, and they don't seem to care- I'd consider telling them this is their 2 weeks notice or whatever is in your contract. Or like lee bee said, have them pick up each time. It's highly unsanitary to have her vomiting all over you and your home. That's something they encouraged, they can deal with it. I'm not sure how you lasted this long!