Oh! My Brother

Only days before their high school festival, popular, outgoing big brother Shiro pushes beloved little sister Masago out of the path of a truck. He dies and Masago, average in ever respect in comparison to her brother, blames herself. In fact, she wishes she had died instead of Shiro. At that moment, she becomes possessed by her brother and from that moment on they're two very different personalities in one body!

(Source: CMX)