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My girlfriend’s older sister is a shut-in. I occasionally see her at her house, and we only exchange greetings, but she’s the epitome of an introverted person who stares at me with a cold, sullen look. One day, I witnessed her masturbating… Her big breasts, slender waist, and huge ass, despite her usually plain clothes, gave me a full erection! From that day on, we started masturbating together in secret, without my girlfriend knowing!