When I returned to my parents' house after a long time, I heard that my step-sister, who was born from my parents' remarriage, seemed to be withdrawn. Worried, I went to visit her. Apparently, Koiro had been teased about her big breasts, and that was the reason she had withdrawn... As I comforted her by telling her that I would do something like a brother, her expression gradually changed... We're step-siblings, so it's okay, I said, and used my big boobs...