The thing is that he has two sisters who love him far not like just sisters, and the modest student constantly becomes the witness of their erotic tricks.
And if only they showed their feelings to him just in home, but they keep on doing it if they have a chance, which is causing some troubles, too.
It's also should be mentioned that the whole situation makes Sora an object of the brutal jealousy from his classmates and his strong reputation as a pervert was stuck in the classmates' minds.
So, where sisters' attempts to seduce their own brother will take them?

[From Steam]