Their secret lingers in the air like an unspoken promise. The sister, lost in her own world, unaware of her brother's lustful gaze. He watches, hidden, as she talks on the phone, her voice echoing in the room. His eyes trace her curves, her full lips, imagining them wrapped around his cock. He can't resist. He slides his hand into his pants, stroking his thick, throbbing dick. She catches him, but instead of anger, there's a spark in her eyes. She listens to her boyfriend's voice on the phone, then whispers, "Fuck me. Make me come." He pushes her down, her ass hitting the desk. He pulls her panties aside, his cock sliding into her wet, waiting pussy. She moans, trying to stay quiet, her body tensing as he pounds into her. He grabs her tits, squeezing them hard as he feels her orgasm building. He pulls out, his cock glistening with her juices, and cums all over her pussy, marking her as his.