I think that Sansa and Arya friendship was destroyed by Septa Mordane(one of reason, why I hate Septa)(and Jeyne Poole).Jeyne called Arya Horseface and bulled her, but Arya was offended at sister. I think, that Arya thought bad about her, because Sansa and Jeyne was best friends (and can condemned sister in Jeyneâs behave, for the same reason.) And, maybe, the fact, that they are friend some time was painful for Arya. BUT THEY WERE KIDS (itâ not Jeyneâs fault on 100%). (And maybe Arya and Jeyne some time jelous of Sansa, such us Arya her sister and Jeyne her friend). Septa, adult women, compared sisters and to exalt Sansa when she (Septa) humiliating Arya. Of course, Arya will be jelous her sister and accept her as selfish, spoiled girl(and as enemy?). Also in first book, when we saw them together it was after Ladyâs death and when in Pov was Septa (or Jeyne). But almost all fandom hate Sansa and behave like only she bulled Arya. Yes, Sansa can called Arya Horseface when younger girl did something bad for her or when Arya offence her. But Arya isnât angel too. THEY WERE KIDS. Septa must learn them, but she more destroy friendship. And Catelyn with Ned were supposed to help them become friends but didn't.