It was their parents

A simple look was all that it was needed to confirm that the pair of voices was indeed his parents. The great Ned Stark, his and Arya's father was sitting by the pond beneath the ancient weirwood, their ancestral Valyrian sword, Ice, on his slap. The auburn-haired lady, their mother, was bent over to his level, talking to him about something with a strange smile he'd never seen her use before. From that distance, he couldn't make out the subject, neither his sister, bent over below him.

Bran gulped, as he felt another wave of temptation seeing his sibling in such position.

But it was what he saw next that should make him truly gulp. Hearing a giggle from his mother that made him think of Arya, the boy watched as their mother began to undress, her big pair of tits, which resembled Sansa's, bare for all to see.

Bran barely had enough time to be impressed before the current head of their dinasty embraced the noblewoman and began to take her against a tree.

There was his answer about his ancestor's using the godswood for more than prayer.

Catelyn Stark moaned in a low volume, but loud enough that they could both hear. His father slamming confidently against her arse.

Watching their parents like that obviously made his manhood hard as stone, despite it being their parents and his mind telling that he really shouldn't be watching.

Entranced, he couldn't really look away. It was his sister's voice which brought him back to reality.