In the dimly lit room, the sister's eyes widen with anticipation as she quietly opens the door, revealing her brother's naked form. His cock stands erect, glistening with pre-cum as he strokes it with a fervor that betrays his usual stoic demeanor. She watches, biting her lip, as he grips his shaft tighter, his breath hitching with each thrust of his hips. The sight of his pleasure sparks an intense longing within her, her own hand drifting between her legs, fingers dancing over her clit as she fantasizes about replacing his hand with her own, feeling the heat of his desire against her skin.