Nee, Nee? Dou Suru!? ~Nami to Mina to Tokidoki Boku~

The main character has an older sister, Minami, who is also his homeroom teacher. One day, a mysterious phenomenon happens to them. Her body is divided into two bodies, and becomes two girls, Nami and Mina!? Their strange cohabitation starts now, involving their neighbor, Hisoka. What will happen to them in the end?