The brother, ever the observant one, catches his sister's gaze lingering on his crotch. A smirk plays on his lips as he shifts, allowing his cock to strain against his pants, the outline clearly visible. His sister, emboldened by his lack of shame, continues to touch herself, her pace quickening as she watches him. He finally looks at her, his eyes dark with desire, and with a slight nod, gives her the green light to continue. She moans softly, her fingers moving faster, her body writhing in anticipation.