Doh...I think it's time to put yourself first for once. It's hard and you may step on some toes but it will be worth it in the end.

I had a relationship I had to end because they were sucking all the life out me. It was really terrible. It was hard and unfortunately I used just about every excuse in the book, but they eventually got the hint and stopped calling.
I hope things work out with you sooner than later, and your sis gets the hint too! Good Luck.