In the dim light, Mary tentatively reaches out, her fingers brushing against her brother's throbbing cock. He gasps, his hips bucking slightly. Encouraged, she wraps her small hand around it, feeling its heat and stiffness. Meanwhile, he gently parts her thighs, his fingers finding her wet, pulsing vagina. They explore each other tentatively, their breaths ragged, their hearts pounding, as they indulge in their forbidden desires.