I Won't Forget the Magical Girl(2011)

Yuya Kitaoka (Ryuki Takahashi) is a high school student. One day, his mother suddenly introduces his younger sister to him. Her name is Mirai (Risa Taniuchi). She possessed magic powers at one time, but does not anymore. Mirai is a lonely girl. Yuya is confused by the situation, but they live together as brother and sister. Mirai gets used to her new life, but ..