Welcome to Psych Central (PC) annoniem9648 . Sorry to hear you are in conflict about your family and possibly breaking contact. You seem to imply your father may have done something to your sister, but breaking contact is a radical step that can distance yourself and make reconciliation difficult in the future if the situation is not what you think.
Have you ever felt at risk with your father? Perhaps your stepmother might be suffering some abuse. Can you talk to her at all? How strong are you emotionally to talk to your sister and see if she will confide in you to a small extent? Is she capable of that?
These are all questions that pop into my mind that preclude giving any advice, plus I find it better to have people make their own decisions. If you have a therapist, some people at PC will find a therapist will help things out.
Some people find the forums give them the compassion and empathy they seek.
http://forums.psychcentral.com
Please feel free to private message any of the Community Liaisons by left clicking on the name in blue to the left of their post) for questions or just to share.