Ichigo Aika: Strawberry Elegy

The series follows Kouta, who gets a new younger sister named Aika after his father remarries. Aika is a gyaru with a dual personality: she acts kindly to her parents, but shows a brattier side to others, including Kouta. Aika quickly develops a fondness for her new stepbrother, and the two form a sexual relationship. (Source: Wikipedia)