It is nothing more than a second. A second that leads her to realise that her sister has transformed.
In that second Arya pauses for the mirror and regards her appearance like any other woman; mussing her hair, hips swaying forward just for a moment, lips pursed.. But then she pulls away suddenly, as if her hand had embraced a flame. And she returns to brushing Nymeria's fur just like before, with strong hands and arched shoulders-- like Arya, Arya with the splindly body of a boy and the dirty cheeks, the Arya Sansa had known once, and now, she realised, wasn't there anymore. Because she was near five and ten now and had scars where there once there should have been their Mother's kisses, and spoke in tongues foreign and dark.
So she begins to follow her. Arya busies herself with rebuilding Winterfell. She cracks dirty jokes with the builders and inspects the shape of the stone. And then there are the men-- her sister's brotherhood, men with blackened teeth who stink of sourleaf and ale-- who she always seems to be with.
The trouble was Arya simply didn't realise her worth. Failed to see that underneath the boiled leather and rough words she was still a lady, still a woman of the North, still a woman who needed to be wedded and bedded. She could play the soldier all she wanted, one day she had to return to who she really was. Just as she had been Alayne, so she would always return to being Sansa.
One night whilst they are lying in bed, Sansa whispers to Arya if she's ever been with a man. Arya barely moves.
"Why are you asking?" She said.
"You were gone for so long." Sansa said, feeling the night chill crawl over her skin. "I need to know what happened to you."
"Why does it matter? I'm back now." Arya said, burying her head in her pillow.
The hall roared with fire and song. Rickon sat at the dais, grinning and laughing at the jesters who twirled around him for his name-day. Everywhere Sansa looked she saw friends. Even Arya was wearing the dress Sansa had made for her and looked almost a lady. And Sansa watched. Watched how the men watched, tried to think how the men were thinking, and saw a woman with the North running through her blood and hips made for bearing children. The gold thread of Arya's gown glittered in the firelight and Sansa felt hope, hope that maybe her sister was still pure as snow.
For a few moments Sansa is distracted by Rickon asking her questions, and when her eyes return to the crowd Arya is gone.
Her brotherhood were in the corner, roaring out songs and slamming their tankards on the table. Sansa feels a surge of panic but manages to remain calm as she rises, hands smoothing down her dress as she excuses herself for the privy.
She follows her feet, the smell of smoke, anything. She feels it in her bones, stirring, an old truth: she is not the sister you remember.
She finds them there, Arya and he shrinking in furs, the torchlight only enough to outline their faces. The bastard boy. She should have known from the way his eyes followed her sister in the training yard, the conversations they had which always made Arya laugh in a way that Sansa recognised from happier times, that it would be him.
"Shh." Ayra giggles, before the Bull Boy nuzzles her neck. Sansa's breath stills in her chest. I should leave, leave. But then before she knew it his hands were moving inside of her sister underneath all those furs. Arya's neck arches and her mouth gasps, as if there was a thirst she needed satisfied. And Sansa grows warm, warm with the knowledge that what she was seeing she was seeing was forbidden, the dark flower of desire, everything she shouldn't want but did.
The Bull Boy thrusts into her sister, grunting like an animal. And the warmth in Sansa only grows stronger as she watches Arya gasp and shiver, and for a moment she swear she can feel him in there, his cum trailing down her thighs and leaving her red raw, cleansed from her desires. It is over in minutes. He helps Arya down, kissing her as they hurry to dress and make themselves Arya and the bastard boy, not the lovers in the dark.
That night she watches Arya sleep and remembers the way he had bit her lips, the way Arya's hands had wandered over his back, leaving trails of red scratches down him. In the morning she would brew Arya some Tansy Tea and make her bathe between her legs, to draw him out of there. Beside her Arya stirred, moaned, and the warmth in Sansa turned to ice.