If there were anyone capable, it'd have to be the Avatar. But there's no way it's her, Korra's the kindest, bravest, most selfless person I know. She would never harm any of us, and she would absolutely never do this when Asami's asleep and could be a risk to her.
[...] Besides, she's been in the castle with us, she would have to be outside keeping the snow up. [Which is a lot more of a logical reason to doubt her guilt.]