Koutei Gentei Manga

From JShoujo Scans: Mio and Ao are twins and they resemble each other in almost everything, beside the fact that Mio is a girl and Ao is a boy. They get along quite well and they always support each other, so when Ao tells Mio that he wants to become a member of LUNA, a group of young models, she is very excited. But unfortunately Ao can't attend the casting so Mio cuts her hair and decides to impersonate her brother for the casting. Though at the casting they immediately realize that "Ao" is actually a girl, they decide to let her...