Sorry, a little late on this, but now that the Gerda thing has been established...
I don't mind OUAT twisting fairytales to suit their stories, but in this case, they aren't really. They are using FROZEN "as is" with almost no character alteration (none allowed, I think is the reason). So they've inserted a non-fairytale into the fairytale world. FROZEN isn't even a classic children's story, like Peter Pan, that has had it's own share of variations and adaption. It's a completely original story written for a film that has nothing to do with any folktale, fairytale, classic kids' tale, etc.
Don't get me wrong, good film -- but somehow out of place in the OAUT world, at least for me. Especially since they have taken the film and basically inserted it with very little alteration or twists.
Of course, now they're pulling stuff from the original tale for little tweaks, like using the Gerda name (NOT the mother's name they used on the gravestones/runic stones in FROZEN, apparently). I don't really mind the mirror thing -- that's a good addition that was left out of FROZEN (well, like EVERYTHING from the orig. story except for the fact that there was a female character who could control ice and snow). Even adding in the Snow Queen -- okay, that's an interesting twist. But Gerda and Kai should have been left out of it. They were never royalty and their story could have been kept separate from this one easily enough. (The orig. story could have sort of happened with this Snow Queen in some other land/time/universe. Making Gerda a princess -- ugh). Just my opinion.