Cherry Works ~Boku ga Imouto o Daita Riyuu~

A guy went with his step sister to see about joining the art club. What they came upon were the club members having fun. To keep their secret, the club president decided to make the brother/sister's rumored close relationship closer. To do that, he will teach them how to have fun. How well he can do that will have different results.

[From Li's Fun House]