I think you are right about letting your sister go until she comes to terms on how much anger she feels towards your parents...it sounds as if it far less threatening for her to use you as a target rather than your parents. In the past my sister has said some pretty creepy things that made me realize how deep her pathologically was...I don't even think a therapist could get at it since it is so embedded.
I think you will find it easier to let go than you imagine...
Bobby