Anetachi ga Zenin Juudo no Brocon datta

The protagonist becomes stepbrother to three women from his father's second marriage. The youngest, Natsuki, is seventeen and a high school senior. The middle sister, Akina, is twenty-three and holds a position as a university associate professor. The oldest, Touka, is twenty-seven and works as an adult professional. Each has distinct interests and personalities, but they all share an intense interest in brocon culture.