Nanako and Nanao: the Day They Became Sister and Brother

七子と七生 ～姉と弟になれる日～

Also known as Nanako to Nanao: Ane to Otouto ni nareru Hi

Nanako is an ordinary high school girl who lives with her mother, who is a taxi driver. One day, suddenly she has a younger brother - Nanao, the son of her late father's mistress. While Nanao's mother is being put in jail, Nanako's mother decides to take care of him. Nanako doesn't like it at all. Everything Nanao does irritates her. Shortly afterwards, Nanako's mother becomes very sick. In fact, she doesn't have much time left. Since she is in hospital, Nanako is forced to live with Nanao alone...