Just watched the episode, what I was interested by was the screwed up sibling relationship between Margaret and Rose. We don't know the overall circumstances surrounding Margaret's drowning, whether Rose was there, if the two of them were playing around the pool, or what. But what followed was years, possibly decades where Rose was actively warding her elder sister's spirit from the only family and home she had, to keep her from doing harm; and it ends with Rose sacrificing her life to become a spirit and neutralise Maggie, or at least to keep her happy. A subtle parallel to Dean and his promise to Sam, I think.