Me, who became masturbation material for my girlfriend's big-breasted, NEET older sister - Rin Yami FNS-176

My girlfriend's older sister is a shut-in. Even when we occasionally meet at her house, we only exchange greetings, but she always looks at me with a gloomy gaze, as if gloom represents that kind of person. One day, I witnessed her masturbating... I became completely aroused by her erotic body—her large breasts, slender waist, and huge buttocks—despite her usually plain clothes! From that day on, we secretly began a relationship of mutual masturbation!