Alberto, entranced by his sister's unknowing display, finds his hand wandering to his crotch. He rubs his bulge, feeling it grow harder as she fingers herself, her moans filling the room. His cock aches to be inside her, to feel her tightness around him. He pulls it out, the sensation of his hand on his bare skin sending shivers down his spine. He strokes faster, his breath coming in ragged gasps, until he can't hold back any longer. He groans, his cock pulsing as he imagines painting her insides with his warm, sticky cum.