I'm living alone in Tokyo in a tiny apartment and my little sister (Kokoro) and I suddenly started living together. Before I knew it, Kokoro had grown big breasts, and she was so defenseless at home that she was always accidentally exposing herself, so I didn't know where to look! I couldn't resist and forced myself on her and came inside her... But that night, Kokoro said, "I've been horny for a long time too..." and asked me to have sex with her and cum inside her! We've entered into a secret relationship that we can never tell our parents about...