I think maybe the girl in the photo who was murdered was her older sister. Just a guess, becasue the film really does not disclose who she is. But it seems also possible that she herself was a victim of a pedophile when she finds the disgusting photos she looks genuinely offender but more than just offended she appears to be hurt as though it happened to her. Those are my two theories. I would be more intrigued if she were the younger sister of that girl they murdered because it would fit with her M.O. of having a sister drive her everywhere. Maybe she feels guilty for being envious that she was older.