Somebody needs to put a sword in the belly of Little Finger. That goofy smirk is really starting to annoy me and the wife. What is his game damn it. What is Sansa beef with Arya in the scene from the bedroom? WTF is Little Finger doing to setup Arya, does he feel threatened at her presence?
I think I have figured it out. I saw a screenshot of the not that the Maester gave to Littlefinger. It was a note, written by Sansa, sent to Robb, in support of Joffrey and the Lannisters, and asking Robb to swear allegiance to them (when she was betrothed to Joff).
Littlefinger is trying to turn Arya against Sansa because "chaos is a ladder", and because he wants to manipulate Sansa and knows that Arya will prevent it.
The conversation between Sansa and Arya was HBO's way of laying the groundwork to make the upcoming rift believable. Poor writing if you ask me, but that is becoming par for the course.