The gym's air is thick with tension and sweat as siblings push their bodies to the limit. The brother's powerful thrusts on the leg press machine catch his sister's eye. She watches, licking her lips, as his cock strains against his shorts. He catches her gaze, smirking, and beckons her over. She straddles him, feeling his hardness through their thin workout clothes. They fuck passionately, their grunts echoing off the empty gym's walls.