The unexpected knock at the door revealed not a familiar face, but a shocking sight that left Essa's heart pounding. Her sister, kneeling on the bed, was completely exposed, her round ass up in the air, and her fingers buried deep within her glistening pussy. Essa's mouth gaped open, her eyes wide with surprise and a sudden, unexpected surge of lust. She quickly composed herself, closing the door behind her and leaning against it, her breath ragged. "What are you doing?" she managed to ask, her voice barely above a whisper.