How did three Arendelle sisters become one?
Via flashback, this pressing question was asked and answered on Once Upon a Time Season 4 Episode 7.
We learn early on that Ingrid initially unleashed her magic when playing with siblings Helga and Gerda because she had to protect them from a creeper.
As symbolized via the ribbons on their wrists, the sisters made a pact on the spot to always be there for each other.
However, after eavesdropping on her dad’s 70th birthday party and seeing Helga courted by the Duke of Weselton, Ingrid decided she should leave.
Gerda says they should at least seek the assistance of a mighty sorcerer in Mist Haven, which leads them to Rumplestiltskin.
As a first line of defense, he offers a pair of gloves for Ingrid to wear. And as a fail safe, he supplies an urn in which to trap her. In exchange, he asks for the sisters’ special ribbons.
Back home, the Duke make a pass at Ingrid, who responds with burst of magic. When Helga gets back, the Duke vows to warn all the realm about Ingrid’s evilness; so Ingrid goes to zap him again, yet fatally freezes Helga instead.
Gerda arrives on the scene and labels her sister a “monster” for this act, trapping her in the urn. Gerda then asks Pabbie to erase the realm’s memories of Ingrid and Helga, as if they never existed. The troll does so, but warns that the truth has a way of getting out.
MEANWHILE, IN STORYBROOKE... Emma and Elsa successfully enact their new spell on The Snow Queen, placing her in shackles and bringing her to the sheriff’s department.
Alone with Emma, The Snow Queen tries to manipulate her by talking about her past in the foster system, but Emma holds strong, even as the Queen brings up Emma’s powers and says her family is scared of her.
But Emma then blows a hole in the building and topples a lamp post on David and she does, indeed, see fear in her parents’ eyes. She departs in her car as a result, with Mary Margaret telling David they have “failed.”
- Regina tells Robin to focus on Marian in order to revive True Love’s Kiss.
- Will Scarlett, surprisingly, offers Robin advice, telling him to stay the course and fight for true love.
- The Snow Queen arrived at Gold’s shop, prepared to make a deal: she gets her ribbons back, in exchange for sharing with him the one “missing piece of the puzzle” he needs to detach himself from the dagger’s grip.
We end the hour with the Queen whispering the secret to Gold and telling him to "do what you want with the rest of the world; Storybrooke will be mine,” to which he replies:
"Not only can I do that… but I shall do it with great pleasure."