What should I do? (Family/Facebook advice)?

I am in no way friends with my oldest sister. Since I am still living with my parents, I am forced to have some sort of relationship with her. She tends to make drama out of anything and because my parents feel guilty for the cirstances of her childhood, they always agree with her in order to avoid drama with her (ie, her picking up and leaving with the grandkids... again.) She will pretend to be friends with you until you do something she doesn't like or disagree with her, then she gives you hell, even to the point of becoming physically violent. Due to these facts, I don't want her to be a part of my life. Since I am of college age, a lot of my life is on Facebook. I have been getting at least one friend request from her every week for the last month or so. I don't accept it and she'll text me to let me know that she sent me a friend request. Although I don't post anything bad that would give her anything to report back to mom or for her to make anything out of, I know that she tells my parents whatever she reads on my sisters', and I don't want her doing that with mine. Today she realized that I recently added my grandma, cousin, and aunt that I hardly ever see and who she sees often. She sent me a text that said "U added grandma, Mindy and Robert but not me on FB! Wow! Thanks!" I don't want to start anything and I don't want to add her to make her shut up either. What should I do?