Arya Stark, Princess of Winterfell

His precious little sister, Arya Stark, lay before him. Her body was athletic from years of running and sneaking around Winterfell (and later, King's Landing), and her bubble-butt could be mistaken for fighting against the thick manhood currently forcing its way inside of it. Anyone who knew Princess Arya Stark of Winterfell as Jon did (carnally and often) would have known better. That butt was made to take him, and when his hips finally collided with her ass cheeks, Jon felt alive and at home.

Of course, Jon was the only one who did know Arya this way. He had known her in this manner (or, had been knowing her in this manner) for years now. It had started innocently enough. Defending her from Sansa's insults. Teaching her to ride a horse and shoot a bow, and then to duel. When Eddard had ridden south to take his place in King's Landing, Arya had come to his room at night, worried that she would never see him again. That had been the first time they had truly crossed a line. When she left his quarters before dawn broke in order to be ready to leave with the King's party, she had had her first kiss, and Jon had held her in his arms until those wee hours. Three special words had been exchanged between them, buried between increasingly fervent kisses, and while those words were ordinary enough ones for siblings (particularly close ones), they took on their truest and fullest form between Jon and Arya. Somehow both knew that what they had was simply meant to be, forbidden or not.

It didn't take long for Jon to realize he had no place going to the Night's Watch. There was a girl out there who needed him. This ought to suffice as an explanation, at least in part, for why Jon Snow was, some years later, embedded to the hilt inside his little sister's posterior.