A scene explaining one of Game Of Thrones' most baffling twists was cut from the finale
http://www.avclub.com/a-scene-explaining-one-of-game-of-thrones-most-baffling-1798632257
As Variety explains (
http://variety.com/2017/tv/news/game-of-thrones-season-finale-cut-scene-1202541941/ ), Arya’s animosity towards her sister was real, and its sudden reversal was supposed to be explained away in a scene that was cut from the season-seven finale:
“We actually did a scene that clearly got cut, a short scene with Sansa where she knocks on Bran’s door and says, ‘I need your help,’ or something along those lines,” Isaac Hampstead-Wright, who plays Bran on the show, says. “So basically, as far as I know, the story was that it suddenly occurred to Sansa that she had a huge CCTV department at her discretion and it might be a good idea to check with him first before she guts her own sister. So she goes to Bran, and Bran tells her everything she needs to know, and she’s like, ‘Oh, shit.’”
So, basically, Arya really did intend to kill Sansa, but changed her mind when Sansa offered up Littlefinger’s throat for the slitting instead. And Bran knew, because he knows everything, but wasn’t going to say anything until Sansa asked. And the show meant to explain all of this, but cut it in favor of Jon Snow’s butt.