Young Wife Sister Swapping

My younger sister, Mika, and her husband, Shun, have moved into the room next to my older sister, Ryo and her husband. One day, Mika sees Ryo taking a man with her and consults Ryo's husband, Yuichiro. However, Yuichiro, who remains calm even after listening to the story, grabs Mika and plays with her. Then, due to her casual behavior, Ryo finds out about her relationship with Yuichiro, and when Shun is seduced as revenge, the sisters and their husband and wife switch partners and start dating...