They need a spanking.

The sisters both stiffened as their Lord Father appeared, eyeing them disapprovingly. He stood there for a moment, silent, and then managed a cough. "I have to say, if this... incident shows me anything it is that I have been entirely too lenient with you two. If Brandon and I had acted like that when we were children, my father would have tanned our hides--and you two are my daughters. I expect so much more from you." He shut his eyes and gave a dull nod. "So... turn to the heart tree, and hitch up your skirts. Both of you."

Arya winced as she followed her father's instructions, as she attempted to ready herself for the pain. That ended when her sister's skirt went up, and she saw the telltale flash of white flesh. It can't be, she thought trying to subtly crane her neck to get a good look. But apparently it was.

"Sansa, why aren't you wearing any smallclothes?" asked Eddard, his discomfort evident.

Sansa's face took on a reddish hue. "I... They aren't very comfortable..."

Arya had to suppress a smirk at that. Her elder sister had acquired a reputation in Winterfell, and this suggested it was well-earned. I hope Prince Joffrey doesn't mind second helpings, she thought with a chuckle.

That cheery thought ended however, when she heard her father step forward and begin to paddle her elder sister's shapely rear. "Ow! Ow! Owww!" moaned Sansa as her father's hand landed on her behind with a dull fleshy slap. Arya bit her lip, and waited, knowing she would be next.