You can report this to you local police and/or district attorney. You seem to think this is something only your sister or your parents can do, but that isn't correct.
Bear in mind that the authorities may disbelieve you for the same reason as your parents. Also, they will probably talk to your sister and parents during their investigation, which may make things difficult for you. But it sounds like no one else is going to report this, so if you're sure about what happened you may want to accept that risk.
Good luck.
