Locating Kelsey

When Olivia discovers that she has a fifteen-year-old half-sister, sitting at a police station and refusing to say a word to anyone, she doesn't want to get involved. That is, until she sees her. Instinctively, she feels this awkward need to protect her and find out everything about the girl. But what she finds out... is far from comforting. (Takes place in Season 13)