Their mother died early, and the two Kashi sisters had to raise each other on their own. The older sister is a very caring person and pampers her younger brother. The younger brother is a high school student - the age of puberty for boys, so sexual desires often arise in him. Having his older sister teach him to study while only wearing thin underwear, sometimes even revealing his private area, this makes his younger brother excited and he has to peek many times, limiting himself and having enough patience. Because he was curious about the woman's body, he couldn't control it and had to seduce his sister. Having only a younger brother, his lustful older sister can't refuse him when he has desires and can only follow her teenage brother's wishes.
