Six years after starting a job and living alone in a rural area, I returned home one day and noticed my younger sister Rinka's breasts had grown significantly. Seeing her like that, I learned she had witnessed our father masturbating and knew our mother refused to have sex with him. Wanting to make her feel good in my mother's place, I began longing to satisfy her. Then, she took the initiative, offering me her body by saying it was fine to touch her breasts if I wanted.