You need to get into therapy about this immediately. You need to talk it through and where to go from here and whether it would be more beneficial to your sister to apologize or if it could harm her. You also need to tend to your own emotional needs. It's very brave to talk about this subject and that is the first step to recovery. As much as you can please try to damper down the emotional negativity and guilt. It did happen and you can't change it but you can start addressing this. Good luck and best wishes.