Mei, my sister, is quiet and gentle, but she was only close to me. My parents don't realize it, but my sister is actually very curious about sex, and I, who now lives alone, have been playing sexual pranks on my sister since a long time ago. One day, my parents decided to return to their parents' home to see how my grandfather was doing after his injury. However, because my sister didn't want to go with them, my parents, who hesitated to leave their daughter of age alone, called me. My sister and I, who were left alone, crossed the line and made love with each other.