Koko dake no Hanashi (MAMIYA Rin)

From Chibi Manga:

Mitsuki has an unrequited love for her older brother, Takashi-chan, since seven years!! And Takashi-chan is a teacher in her school!! "let's take some distance", Mistuki said… she wanted to keep secret the fact that they knew each other.

But that secret will be the start of their love!!