"We'll be as sisters," Margaery had told her. It's true that a marriage to a son of Highgarden will give her the Tyrells as good-family, but Sansa will never give up her own sister. Arya was wild and disobedient and she might be dead in the war or she might have escaped somehow but she's free, free as a bird, more free than Sansa is ever likely to be.