A Sister's All You Need.

The story follows the everyday life of Itsuki, a novelist and "modern day Pygmalion" who works day in and day out to create the ultimate younger sister. He's surrounded by various other characters: a beautiful genius writer who loves him, his big-sisterly classmate from college, a fellow male writer, a sadistic tax accountant, and his editor. They're all looked after by Itsuki's perfect younger step-brother, Chihiro, who has a serious secret.