Arya is just an asshole to Sansa for no reason and she is also getting played by Littlefinger how annoying
Bran will tell Arya and Arya will make it seem like she's onto him. Littlefinger is thinking he'll pit Arya against Sansa, bu it'll backfire because of Bran.
Edit: thats the way I see it. Arya can read lips and she probably knows he's try to scheme against her. Remember she's learned to be in the shadows.