I'm not sure why people keep suggesting Arya killing Sansa is plausible (and I'm not singling you out, ItIsKnown, lots of boarders have suggested this). If Arya really hated her, wouldn't Sansa have made The List? Doesn't she even say to Ned, "I don't hate her, not really" all the way back in AGoT? I suspect if they do meet again, they will both be crying with joy to see each other again. Bickering was for the summer. And yes, it's clearly revealed that a FM cannot be given a kill assignment where the target is someone he/she knows, even a bit, which is the whole reason behind the sailors in AFfC making Arya repeat their names over and over again when they're on the way to Braavos. Arya can't be assigned to kill any of her former friends/enemies/family members, although I could see her hiding the fact that she knows someone on her list in order to be able to kill them.