I started living with Yuri around last winter. Today, Yuri's sister Marin had to go on a sudden business trip. Unable to find a hotel, she stayed at our house for a week. Perhaps because Yuri was here, she was very casual, always in a 'defenseless' state, revealing her cute little panties and her big round butt in front of me. Especially after taking a bath, Marin comfortably walked around the house naked, allowing me to inadvertently admire her model-like beautiful figure. Unable to resist her anymore, the next morning I turned on a porn movie and masturbated alone in my room. While masturbating, Marin suddenly walked into my room and witnessed everything. But instead of leaving, she came closer, took my phone, and watched the porn movie I was watching. Feeling extremely aroused, I no longer cared. I rushed forward and possessed this exquisite naked body! At first, she resisted, but as time went on, she moaned louder, matching my movements, and reached climax over and over again! And our twisted relationship began from there, even having sex next to Yuri because both of us felt even more aroused that way...