As the siblings huddle together for warmth, their bodies betray them. The brother's cock stiffens, pressing against his sister's thigh. She gasps, feeling the heat through her thin pajama shorts. They freeze, eyes locked, breaths ragged. The sister, her heart pounding, reaches down and strokes her brother's rigid length through his pajama bottoms. He groans, his hips bucking slightly. Emboldened, she pulls down his pants, revealing his throbbing cock. She wraps her small hand around it, feeling its heat. He hisses, his fingers finding her wet pussy through her shorts. They're crossing a line, but they can't stop, their bodies driven by a lust they can't control.