[s5e19] Sisters -

: When Cora realized their bond could interfere with her plans for Regina to become Queen, she used water from the River Lethe to erase their memories of each other. Present Day: The Underworld Conflict

: After Regina was injured playing with magic, a young Zelena was brought in to heal her. They bonded instantly, unaware they were sisters. [S5E19] Sisters

The episode primarily deals with the "unfinished business" of Cora Mills in the Underworld, specifically the rift between her two daughters. Flashbacks: A Forgotten Connection : When Cora realized their bond could interfere

: Cora apologizes to her daughters, telling them that love is not a weakness but a strength. This act of "finishing her business" allows her to pass into the light rather than being dragged to hell. Subplot: The Twin Showdown The episode primarily deals with the "unfinished business"

: Fearing Zelena’s romance with Hades, Regina enlists Cora (now in the Underworld) to help separate them.

There are other series with a Season 5, Episode 19 that feature "sisters" or similar themes, such as: Once Upon a Time Review: Sisters (Season 5 Episode 19)

: Cora initially tries to trick Zelena using "forgetful water" but eventually has a change of heart. She restores their childhood memories to break the cycle of hate.