In the dim light of his room, the boy's eyes fall on the discarded panties, the crotch darkened with his sister's arousal. He picks them up, pressing the damp fabric to his face, inhaling her scent. His cock twitches, and he quickly undoes his pants, letting it spring free. He strokes it slowly, building a rhythm, his mind filled with images of his sister, her curves, her wetness. He imagines her in those panties, feeling the fabric against her pussy, then feeling his cock there instead, pushing in, filling her up. His strokes become more urgent, his breathing ragged, until he can't hold back any longer, his cock pulsing as he comes into his sister's panties, staining them with his own desire.