Which leads us to this conclusion: what if Arya is has already killed Littlefinger, and is testing Sansa's loyalty to the Stark family? He's not on her list, and while she's impulsive, she certainly isn't stupid. Arya would certainly be able to see that Littlefinger is stirring the kettle. After all, middle Stark trained with the Faceless Men, there is no way that Littlefinger is more stealthy or cunning than she is. That creepy conversation she had with Sansa was her way of lettering her sister know that she's steps ahead.