One day, they suddenly became brothers and sisters. A red stranger whose blood is not connected even though he is a brother and sister. While I still can't become a "family", I'm alone at home. My brother-in-law, who couldn't stand it because he was lustful for his cute little sister who spends his time unprotected with a kotatsu, is mischievous in the kotatsu. My younger sister also became interested in it and committed a bold crime. Includes 3 brother-in-law and sister S ○ X.