The siblings huddle together in the dim light, their bodies close, breaths shared. The older sister, her voice barely a whisper, tells her brother to stay still. She slides her hand down his chest, tracing the lines of his muscles, her fingers brushing against the bulge in his pants. He inhales sharply, his cock twitching. She looks up at him, her eyes filled with a mix of fear and desire, then slowly unzips his pants, freeing his throbbing cock. She wraps her hand around it, feeling its heat, her own body responding to the taboo touch. They stand there, frozen in time, their hearts racing, as they cross the line they've been taught to never cross.