This post contains spoilers from Game Of Thrones Season 7, Episode 5. Last week's episode of Game of Thrones contained one of the most interesting scenes of the season so far: Littlefinger giving Bran the knife that was once supposed to murder him as he lay in a coma. Sansa's alarm bells went off immediately. Littlefinger was playing at something, even if he failed this round. Now it looks like he's trying again with the new Stark on the block, as we see with the note Littlefinger left Arya from Sansa.
It starts out innocently enough, with Arya not trusting the man who gave a knife to her brother, and whom her sister is clearly unhappy with. Of course she's going to start spying on him. (Also, what better things does she have to do? Sansa's running the place, Bran's communicating with trees. She can't fight Brienne all day.) Littlefinger wasn't born yesterday, and he knows a spy when he sees one.
And since it's obvious Sansa already is troubled by her last week, it's only in Littlefinger's interest to play up that disharmony as best he can. If he can convince Sansa and Arya that they are working against each other's interests... but how? Arya already suspects Sansa of wanting power.
Remember that double-take Littlefinger did the other week when Maester Wolken said Luwin kept copies of everything that got sent to Winterfell? Apparently, that includes some less than fortunate ravens that came from King's Landing during Robb Stark's war.
And, if you'll remember, back in Season 2 when Sansa was still engaged to Joffrey and a prisoner of the Lannisters, they had her sign and send a few missives, once that basically commanded her family and the "King in the North" to give up the fight and surrender.
Littlefinger figures that Arya isn't going to take too kindly to any ideas that Sansa might be working for a power grab. So, he leaves that missive with her treasonous statement against "The King in the North" right where Arya will look for it in her spying. Turning the sisters against each other looks like it's going perfectly to plan, no?
Will Arya confront Sansa about the missive? Will Sansa realize what Littlefinger is doing? Can Bran point out Littlefinger's guilty of betraying their father and setting him up to be beheaded?
That would be a really useful thing to point out, Bran. Just saying.