^ I agree
I think that Arya was impressed with Brienne's fighting skill when she saw the fight with the Hound... and now she is very good with the sword, so she wanted to see how she would do against Brienne
I love those smug smiles that she had
I think Sansa was just overwhelmed with how well Arya was with weapons, probably imagining what she had gone through to actually get so good at it... and also, realizing that the kill list of hers is actually true. At least that's what I'm hoping for, I really don't want them to have any sort of disagreement or jealousy or anything bad happening with them so quickly after their reunion