Futago, Futagokoro.

A boy involved with one sister faces complications due to the other. Haruta is in a relationship with Mafuyu, a quiet and reserved girl, and experiences his first taste of a romantic high school life. However, his contentment is disrupted by the arrival of Mafuyu's twin sister, who is bold and completely different from her. As her presence grows, the balance of his relationship begins to shift.