A simple idea...
Brother's and sister's rooms are next to each other, separated by their closets. For some reason, the closet doesn't have a proper wall between, just a single layer. Brother finds a hole in the wall that he had never paid attention to before and looks through at his sister. Because of the clothes, he can't see much. However, being a perverted, horny teenager, he decides to prank his sister by sticking his cock through the hole when she is looking for something to wear. To his surprise, she grabs him and gives him a handjob. Things escalate to other uses for their glory hole. She knocks lightly on the closet wall to tell him she wants him to use the glory hole and he obliges. This turns into a long relationship. The final surprise he finds is that his sister drilled the hole because she is also perverted and horny and wanted a view of him.
