I take it that your sister is your parents guardian and caretaker now? if so theres not much you can do unless you go to court and show how being at your sisters is harming your parents and the court appointing you as your parents care takers. other wise you will probably have to find a way to visit with your parents while following the rules that your sister sets down, there may be reasons that you are not in on that is causing your sister to limit your contact. another suggestion is sit down with your sister and a lawyer, maybe some of the problems can be fixed in a way that you be legally awarded visitation with your parents. whether that visitation is supervised or not would be up to the courts