Spoilers for "Once Upon A Time" past this point.
We've previously established that the villainous Snow Queen (Elizabeth Mitchell) may, in fact, be Elsa's Aunt, and the source of all her icy powers. And after Anna (Elizabeth Lail) visited the rock trolls (yep, those of "Fixer-Upper" fame), that particularl family lineage was confirmed: The Snow Queen is the sister of Elsa and Anna's mother, Gerda.
Originally named Ingrid, she was wiped from everyone's memory, along with... Wait for it... Gerda's other sister Helga. So Anna and Elsa actually have two secret aunts.
The other important detail to know? Emma Swan (Jennifer Morrison) looks just like Helga, though we haven't met whoever is playing her yet - the third sister won't be introduced until the seventh episode of the season.
That said, as Captain Hook (Colin O'Donoghue) quipped, "Spend a little more time in this town, love, and you’ll realize just about everyone is related." So just who is Helga? Because she's not just a random third sister who died and was wiped from everyone's memories: she has to be connected to everyone somehow.
This is probably the most likely possibility, as we know the "Sleeping Beauty" big bad will be the villain in the second half of the "Once" season. We've only seen her a few times on the show, played by Kristin Bauer van Straten... Though she's one of the few recurring characters we've never gotten a backstory for.
She's The Sorcerer
Probably a bit of a stretch, but while "Once" has been running their "Frozen" arc, they've also been running a storyline about Mickey Mouse's magic hat from "Fantasia." We've met the show's version of the Sorcerer's Apprentice, but not the Sorcerer him - or her - self. Could Helga be the Sorcerer?
It's not like the show hasn't dealt with time travel before, so what if the reason Emma looks so much like Helga is that she is Helga? The logic of this makes our head hurt a bit: it would mean that Snow White and Prince Charming somehow had two other daughters other than Emma, making them Anna and Elsa's grandparents... And that the Snow Queen raised her own sister as her daughter, since we got a glimpse that teen Emma had been adopted briefly by the Snow Queen IRL.
So it's a stretch, but on a show where a young Peter Pan turned out to be Rumpelstiltskin's deaged father, who was trying to steal the heart from his great-grandson, who is the adopted son of the Evil Queen and the grandson of Snow White... Anything goes.
Who do you think the third sister is? Let us know in the comments below!