An upcoming episode of ABC's Once Upon a Time will delve further into the backstory of sisters Regina (Lana Parrilla) and Zelena (Rebecca Mader).

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Ava Acres (Crazy Ex-Girlfriend, Agents of S.H.I.E.L.D.) has been cast as the young version of Regina while Isabella Blake-Thomas (The Green Balloon Club) is playing young Zelena, according to a new TVLine report.

Both will appear in episode 19 of this current fifth season, an episode fittingly titled "Sisters".

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The TVLine report further delves into Regina and Zelena as their younger selves, noting that Regina is "neglected and lonely" and has no use for her mother Cora's (Barbara Hershey) royal status. Meanwhile, Zelena is coming into her own as a magical being, while her father dismisses her.

But Zelena won't just be appearing in flashbacks despite being banished to Oz in the midseason finale.

"[Zelena] is absolutely not done," Once Upon a Time co-creator Edward Kitsis recently told the publication. "The Zelena story in Oz, with Dorothy [Teri Reeves], will tie into our Underworld story pretty significantly."

It remains to be seen if Hershey will reappear as Cora in the "Sisters" episode, though the TVLine report states that it is pretty likely.

Once Upon a Time returns for the second half of season 5 on Sunday, March 6 at 8 p.m. ET/PT on ABC.

Watch a promo for the midseason premiere of Once Upon a Time below:

Tags: Once Upon A Time television ABC