Playing the Game of Face - Make a lie sound like the Truth. - The second, that Arya would ever wonder what it was like to wear all of Sansa’s pretty dresses was hint 1). that she was lying. Of all people, Sansa knows Arya doesn’t want to be a “proper lady”, never has. Arya could have a pretty dress made for her like she had her fine light armor made, if she really wanted. But Sansa still went snooping for some stupid scroll instead of coming to her directly. She doesn’t trust her sister enough to respect her by a direct confrontation. And she really buys that Arya would kill her like LF implied!
That last bit irritates the fuck out of Arya – she’d rather have lost the game this time – but again, Arya doesn’t realize Sansa’s brain shut off and she’s mentally back a few seasons. So she tells Sansa “You lost the game” by handing her the dagger instead of stabbing her. She waits for Sansa to take it, and gives her a moment to hold it. She’s giving her the only final clue she can: A non-verbal one. We came here looking for our enemy. I’m not your enemy. Here’s how you find the real enemy – you brought him into Winterfell, and he brought this with him. And remember how you can get proof.“ Of course the plot could have just stopped and started with that reunion, but it would be far worse of a dodge than at least trying to let the audience wonder how things will go down.
And she leaves Sansa with the (all male) faces, as if to say "Here, if you still don’t trust me, here’s some real blackmail material (my precious faces). Freely given. I know you’re smarter than this. I only told you about what I found (the letter) and told no one else, as you must know. I trust you to keep my secret. And it’s one I really will die for telling if you blab.”
At the end it was all a lie. The dagger was turned around, the blade pointed at Arya’s stomach. The pause with Sansa holding it there. This is Arya confiding it was all a lie: Arya would never kill her just like Sansa would never kill Arya. But Arya was also sending two messages: 1). LF is our true enemy and this is his weapon. 2). I told you my secrets, leaving you to hold my dagger, and all my precious faces that includes fucking Walder Frey. I am trusting you not to condemn me and giving you all the evidence to do so. I’m trusting you not to use it. At this point in E07, all the Starks will begin to work together.
This leads to Sansa putting Arya on “trial” after Bran exposes LF’s past lies and plots. Sansa gives Arya back her dagger and she kills LF.