Siblings' guilt-driven closet copulation

A brother and sister, isolated together, give in to their secret desires. She masturbates, unaware he's watching. Caught, they argue, but soon, curiosity and lust take over. Naked, she gazes at his throbbing cock, wishing it inside her. Just as they're about to cross the line, a third party interrupts, leaving them breathless and unfulfilled.

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