Majima Takahiro has two sisters.

His older sister's name is Miyuki. She's a little absent-minded, but she's a kind sister who spoils everyone.

His younger sister's name is Kana. She's cheerful, kind-hearted, and a little shy and timid. She's a great sister who everyone likes.

Though they're brother and sister, Takahiro feels attracted to both Miyuki and Kana as girls.

One day, he hears about idol auditions and applies for them without their permission.
As a result, both of them pass the selection process.
Although they weren't really interested, Takahiro encourages them to become idols.
For Kana, it was her trusted (and favorite) older brother, and for Miyuki, it was her cute (and favorite) younger brother who recommended it, and the two became candidates for the idol group "Se-Kirei."

Takahiro had a plan.
In order for the shy Kana to appear in public, and for the airheaded Miyuki to become an idol, various training is required! Takahiro teaches the two of them all sorts of naughty things under the guise of training!