Fui ni Arawareta, Toshi no Hanareta Gimai no Hanashi

One day, the delinquent boy Ryu was surprised by his estranged father, who made a rare appearance at home. His father left Suiren, a girl 12 years younger, with Ryu, telling him, "This is your stepsister." The life between stepbrother and sister began from there. The two were bewildered by the situation at first, but they will make their way through, on their way to becoming a family...

(Source: Ganma!, translated)