In the heat of the day, I catch my sister undressing, her lithe body glistening with sweat. I can't resist the temptation, creeping up behind her and pushing her against the wall. She gasps as I enter her roughly from behind, her hands clawing at the wall for support. I fuck her hard, my balls slapping against her ass, as she struggles to stay silent. The sun beats down on us, our bodies slick with sweat, as I drive into her again and again, filling her with my cum before collapsing against her back, panting.