Welcome Glimmer: I'm so sorry about your family situation. It sounds like you have been painted black (split). That is a very uncomfortable situation to be in.
My mother started ending our phone conversations with a "you be nice", instead of "I love you".
That is a very invalidating thing for your mom to say. It does sound like your sister has launched a smear campaign on you. My sister has done something similar to me.
If she is living with your parents, or on her own? The only options you have are to visit family when you know your sister isn't there, or become confident with setting boundaries and using some communication tools. A good place to start learning is by going to the large green band at the top of the page and check out some lesions in the "tools" area. You can't change your sister, but you can set boundaries and control the way you interact with her and react to her. You don't have the power to change her.
Check out some of the lessons and let us know what you think.