Can you contact your state's board of education division that handles complaints about bullying not being addressed in this school? I personally know of an incident where a parent did this. The local school superintendent personally sat in on the meeting with all interested parties until the matter was resolved. Sometimes you have to go above local school administration and get state education administration involved to have bullying complaints adequately addressed. Also keep a log of any meetings you have with the school and contact from the bully to your sister. Your sister might need to wear a camera to catch incidents of this student bullying her. Do not engage the bully or the parent because it's your word against theirs.