Depending on original work by Kino Hitoshi.

Arisa and Mika Nanase are sisters and also the school super-idols. Those are the hottest and cutest girls within the school. Sousuke is a normal student that like the rest of the boys, are only able to look at the super-idol sisters from afar. Sousuke likes Arisa but sometimes never hope to even speak to her. 1 day Mika tells him she want to schedule an appointment him after school. Later, he meets with Mika and Arisa and they reveal they have now become siblings! The reality is that Sousuke's father, an adventurer, as well as the Nanase sister's mother, an archeologist, met at an excavating ruin in Brazilian, fell for each other, and married.

Mika, Arisa, and Sousuke are siblings and definately will live beneath the same roof in the future. Sousuke now should be careful failing to get caught masturbating by Mika or Arisa ...or maybe some may give him additional aide... What about having with Mika? ...Siblings do bathe together in the end...