He mounts her

Bran started rubbing his crotch against her butt. He feels his shaft getting painfully harder, the trousers causing actual discomfort. How could just feeling his sister's bottom provoke that reaction on him?

He can't hold anymore and get on top of her. It felt great having her smaller frame beneath him. Her butt was so soft and fragile against his crotch. It was not too big, but not too bony either. Just enough to grab a handful of it.

He kissed what it felt like her neck and her ears, some of her hair getting on the way. It was a way of complimenting her shape. He grabbed the end of her skirt and lifted it until there was nothing covering her arse. He straightened himself, still mounting his sister.

Tired of the fabric restricting him, Bran undid the knot of his trousers. His member sprung forth landing on Arya's bottom. His manhood was twitching in anticipation. Nothing else was on his mind but entering his sister's hole.

The Stark slipped his legs between hers, spreading them just wide enough for him. With his strength he grabbed her waist and pulled it up. A small sliver of moonlight illuminated her backside. The image of the perfect curve of her cheeks and her glistening womanhood imprinted itself on his mind. He was sure then that whenever he'd thought of his sister, this picture would come to mind. His cock has waited long enough...