Spoilers for Game of Thrones.
Arya and Sansa’s recent reunion on Game of Thrones has brought chaos and fear to Winterfell. Little Finger has noticed Arya’s frequent snooping and spying, and he has been working on pitting the sisters against one another.
This is almost certainly, however, playing into Arya’s overall plan. Arya already finds it difficult to trust others, and Little Finger is one of the hardest characters to trust. Arya would likely realize that it was Little Finger rather than Sansa who put the letter in the mattress. Arya hated Sansa growing up for her obedience and want to be liked rather than her cunning and scheming. Besides, Arya has likely noticed Little Finger’s attempts to isolate the Lady of Winterfell. As far as plans go, it’s not the best and vastly underestimates Arya’s abilities (perhaps this was Little Finger’s plan as well. We’ll have to see how that plays out).
Arya telling her sister that they are playing The Game of Faces was accurate. Sansa just didn’t know that Arya has been playing the entire time she was at Winterfell. When she hands Sansa the dagger, she gives her the opportunity to “punish” her for lying. Sansa knows that Arya’s account of their father’s death is false, but she becomes hysterical trying to convince her sister that she was miserable in King’s Landing.
The Game of Faces, though deranged in any capacity, is best left to strangers lying about themselves. Arya’s plan to trick Little Finger into thinking she’s his pawn only serves to gaslight Sansa. While she could continue to perform sneaking around her sister for Little Finger’s benefit, she instead chose direct confrontation. This is in line with Bran’s decision to do so as well to convince his sister that he was the Three Eyed Raven. This tactic was also used by Little Finger, Ramsey, Joffrey, Cersei, and every other horrible person in Sansa’s life.
What purpose is there in continuing to torment Sansa? Rather than allow her to turn from porcelain to ivory to steel, she remains stagnant: a noble lady subject to the abuse of the court. Every time she has the opportunity to grow or take on new responsibilities, another character is waiting to cause destruction. Any other character would have become Queen of the North or died, but Sansa simply remains Lady Stark. There’s fewer and fewer episodes for there to be a payoff for the abuse she has endured. Even in season 7, she continues to be Game of Throne‘s favorite punching bag.