Fui ni Arawareta Toshi no Hanareta Gimai no Hanashi

One day, the delinquent, Tatsu, was brought home a young girl, Suiren, that is 12 years younger than him. His father that rarely comes home tells him that she is his stepsister. Thus begin the life of the two stepsiblings. While both seems confused, they go through numerous trial and error to try to become a real family.