In the cramped guest room, the siblings huddle together for warmth. Her hand brushes his crotch, feeling his growing bulge. He stiffens, but she persists, rubbing his cock through his pajamas. They lock eyes, their breath fogging the window. She whispers, 'We shouldn't,' but her hand keeps moving. He groans, then flips her onto her back, pinning her wrists above her head. She gasps as he grinds his hips, his cock rubbing her clit through their thin clothes, driving them both to the edge of no return.