Fetixxx!

Hashinomoto Maaya has wanted to marry her little brother Rintarou since she was a little girl. Rintarou seemed to feel the same way, claiming that he wanted to be together with his sister when they were all grown up. Now, after going to separate elementary and middle schools, Rintarou is about to enter Maaya's high school! Maaya is filled with excitement for her school life together with her precious brother. But when she finally sees him, she realizes the true meaning of Rin-chan's words all those years ago. He didn't want to marry his sister, he wanted to be just like her. So now, he is beginning his high school life dressed as a girl...!

(Source: MangaHelpers)