Side with Sansa

You take a deep breath, bracing yourself for what's to come, and rounding the corner, you approach the group of women who turn towards the newcomer to their argument.

"Apologies, Lady Stark, I couldn't help but overhear. Arya, you did disappear yesterday for quite some time whilst I was supposed to be looking after you, "you throw what you hope is an apologetic look at Lady Stark before continuing,"it was also just after I noticed the Septa, Sansa and the other girls for lunch, too, which would mean you had ample opportunity to take Sansa's sewing kit without her knowledge."

Catelyn regards you a little longer before turning to her daughters.

"Arya, you will go and retrieve the sewing kit from wherever you've stashed it. Sansa, return to your bedchamber and your studies and Arya will return your things to you there."

With a curt nod to you, the Lady of Winterfell strides off, her youngest daughter trailing behind her.

Sansa turns to regard you, appraisingly.

"Thank you, Jon." She seems surprised you didn't stick up for Arya. Most recognised the special bond between the two of you, and to have broken that trust must have surprised both Sansa and Catelyn, not to mention Arya - she was probably devastated.