Arya was always like this. She never liked Sansa, even before the events of King's Road. Much of the stuff Arya "got away with" was picking on Sansa. Mud in the eye, a stain down the front of the dress, a bed full of sheep shit...and Sansa's objections and occasional eyeroll at Arya's shortcomings was always treated as "just as bad." So what opportunity did Arya ever have to develop any empathy for her? Any concern or caring? When was she ever encouraged to? Sansa's concerns were by definition stupid to Arya. Arya has a conscience, but it only covers things like a sense of honor and being kind to animals and responsible for her servants like Mycah and Gendry. Sansa doesn't fit in the picture because although Arya wants to be a fighter, she never put Sansa in the picture as "lady to be protected." She also doesn't have a lot of empathy, and she has even less of it for Sansa. Sansa can't do the things Arya can do, and that inspires disgust and annoyance in Arya, not protectiveness or concern. Sansa can do a lot of things Arya can't--make clothes, ration food, keep track of groups of people...and this inspires annoyance, too, not admiration or respect. Arya never really saw Sansa as anything but competition. They really aren't in competition for anything now, but the habit is still there. It hasn't been replaced with anything.