The creaking floorboards overhead signal her presence, but he doesn't stop. His sister-in-law is just upstairs, blissfully unaware of the solo session happening below. His hand moves faster, his cock throbbing in his grip. He pictures her, her smile, her laughter, her body. His heart races as he thinks about the taboo, the thrill of it all. He can't hold back any longer. With a silent groan, he cums, his body tensing as he releases, his mind filled with thoughts of her.