(((Anne))) I think your sister has taken advantage of your kind and forgiving nature...you let her get away with the abuse and so she takes advantage of it. She can "count on you" to react..it gives her power over you in a strange kind of needy way...in my opinon.
that's not a nice way to treat anyone, let alone a family member. Like Moose said, I'd distance myself from her...you have enough grief in your life without taking on more.
Just because we are related to some people, we don't have to like them.
And we certainly don't have to be their whipping board.
You deserve better.