People also aren't mentioning the fact that cutting ties with your family is haraam too! Maybe you should mention to them that what they're doing is haraam, and it's hypocritical for them to do so something haraam and then claim that your sister is leading a haraam lifestyle. If you show them that you're capable of not following her footsteps, then inshaaAllaah they might allow you to see her. Make du'a for both your parents and sister inshaaAllaah.