Regina has an unpleasant family reunion on the latest installment of "Once Upon a Time."

In "It's Not Easy Being Green," the heroes pay their respects to Neal (Michael Raymond-James), who met his untimely death at the hands of the Dark One.

Zelena (Rebecca Mader), who is feigning an innocent midwife persona, visits Granny's Diner to ask if she missed the wake. She then challenges her little sister, Regina (Lana Parrilla), to a showdown that night on Main Street.

Regina doesn't believe that Zelena is her sister until she realizes that a letter she thought Rumple (Robert Carlyle) had written to praise her was actually written to Zelena.

Zelena arrives on Main Street before Regina, and is about to wreak havoc when Regina finally shows up. Regina smacks Zelena in the face, then Zelena fights back by tossing her into the clock tower.

Zelena is about to take Regina's heart, but realizes that she's hidden it somewhere.

Later, Regina unearths her hidden heart, and entrusts it to Robin Hood (Sean Maguire), the man who happens to be her one true love. Regina realizes that she needs to embrace their budding relationship.

Meanwhile, in a flashback from the Fairytale Land That Was, Zelena is told by her adoptive father that she was found in a basket. Zelena then visits the Wizard of Oz, who tells her that her mother, Cora, gave her away as a child.

Zelena gets angry that Rumple is teaching Regina, so she goes to visit him. He's impressed with her powers, but she begins turning green with envy when she hears that he's continuing to teach Regina. She tries to kill her sister, but finds that Regina is actually Rumple in disguise.

Zelena tells Rumple that she has special shoes, and can take him to a land without magic. Rumple then tries to kill her but she's too powerful, and is able to evade death.

She then goes back to the Wizard of Oz and turns him into a flying monkey.

On a future episode of "Once Upon a Time," fans will find out the identity of Zelena's father. TVLine hints that Zelena's familial history will be unearthed in the episode that brings back Rose McGowan as a young Cora, as well as Eva Borne and Eric Lange as young Eva and Prince Leopold.

"We're flashing back to Leopold's younger days to fill in more Regina backstory," Ginnifer Goodwin, who plays Snow White, told TVLine. "[Revisiting] Snow White's childhood and her parents can take us there very easily, to show how the Evil Queen's family, how those royals intertwine with my family's royal lineage."

And on this Sunday's episode, Hook (Colin O'Donoghue) will encounter another famous pirate: Blackbeard.

"Blackbeard obviously is very much a dreaded pirate, and you get to see Hook interact a little bit with him - I can't tell you how or why, because that will give away too much - but and it harks back a little bit to all the old Errol Flynn films and stuff like that. It's good old-fashioned, swashbuckling fun," O'Donoghue said.

On the next episode, "The Jolly Roger," "Ariel returns to Storybrooke and pleads with Hook to help her find Prince Eric, who never returned to the town when the new curse was invoked. Emma agrees to let Regina teach her how to use magic so she can help defeat Zelena, and Mary Margaret and David try to prove that they can be just as much fun as Hook is with Henry- who thinks they are boring. Meanwhile, in Fairy Tale Land during the past year, an angry Ariel confronts Hook over her missing Prince Eric, who she assumes was kidnapped and possibly killed by the swarthy pirate. But when Hook confesses that the Jolly Roger has been stolen and Eric is most likely the thief's prisoner, Ariel unknowingly provides him with a clue as to who the culprit is, and Hook--with Ariel in tow--goes off in search of his ship."

Watch the promo for the next episode below, which airs Sunday, April 13 at 8 p.m. EDT on ABC.