As the siblings huddle together, their bodies pressed close for warmth, they can't deny the growing bulge in his pants or the dampness between her thighs. The brother tentatively reaches out, his fingers brushing against her nipple, feeling it harden at his touch. She gasps, her head tilting back, exposing her neck to his hungry lips. His hands roam her body, cupping her ass, squeezing her breasts, while she grinds against him, her moans filling the small space. Their clothes become barriers they can no longer tolerate, and they strip each other, their eyes wide with anticipation and fear, ready to cross the line that can never be uncrossed.