I Kissed My Girlfriend's Little Sister ♥

Hiromichi Satou has been single his whole life, but, he recently got a girlfriend named Saikawa Haruka, and they have been dating for a month. His father remarries, and he gets a new step-sister Saikawa Shigure, the identical twin of Haruka. Sato and Shigure agree to keep their step-sibling relationship a secret from everyone, and Haruka. How will the story unfold?