The siblings stand under the cascading water, the steam swirling around them. The brother, emboldened by the anonymity of the online challenge, reaches out, tracing a line down his sister's back. She shivers, her eyes fluttering closed. He takes this as an invitation, his hands exploring her body, soaping her breasts, her stomach, her thighs. She leans into him, her breath hitching as his fingers graze her pussy. He turns her around, his hard cock pressing against her ass. Their eyes meet, and in that moment, they both know they've crossed a line, but the heat of the shower and the thrill of the taboo are too much to resist.