Incestuous lust under the same roof.
With the coast clear and the house to themselves, the siblings can't help but act on their taboo desires. He's been fantasizing about her, his own sister, and she's been sneaking glances at his bulge, wondering what it would feel like inside her. They start innocently enough, watching TV together, but the tension is too much. He makes the first move, his hand resting on her thigh. She doesn't pull away, instead, she leans into him. His hand moves up, under her skirt, finding her panties soaked. She gasps, then turns to him, kissing him deeply. They undress each other, their bodies pressed together, their hearts racing. He enters her roughly, her back arching, her nails digging into his back. They fuck hard and fast, their moans filling the silent house, their bodies covered in sweat and lust. They come together, collapsing onto the couch, their bodies entwined, their secret safe for now.