It seems she is keeping things from you guys, in fear of involving your dad. The only way i can think of getting her to reveal everything is with the actual threat getting your dad involved. It maybe emotional blackmail, and may even backfire, but at the moment, it seems the only way to get to the bottom of a serious issue. You have to think in the long term and that is the saftey of your sister, physically and emotionally.
Secondly, should it backfire, you should definately get your dad involve, as he seems the only person strong enough to take action either way, and she knows that.