My first thought was that it was her sister that she lost. As in their “sisterhood”, not as in her “death”. But earlier when they had their falling out that made her hate her. Maybe they were close once. I’m not sure if it’s a realistic depiction of a sibling bond, but it’s something that would fit into the META of the show. It’s what I would do if I were a writer when trying to spin the story around a central plot point.