Goshuushou-sama Ninomiya-kun

Ninomiya Shungo, a male high school student trained in advanced hand-to-hand combat, lives in a huge house with his older sister Ryouko, who is almost always gone on some dangerous mercenary mission.

One day, his sister sends Tsukimura Mayu and Mikihiro, two siblings, to live with him to help Mayu overcome her fear of men.

Unfortunately this is accomplished by making Mayu and Shungo share the same room, bathe together and even sleep together in the same bed.

Making things even worse, Houjou Reika, the student-council chair, is blackmailed into becoming their maid due to a video that Ryouko recorded, in which she tries to seduce Shungo.