Nakaimo - My Sister is Among Them!

Kono Naka ni Hitori, Imōto ga Iru! (この中に１人、妹がいる!, My Little Sister is among them!?) is a Japanese light novel series by Hajime Taguchi. It was adapted into a manga in 2011 and an anime television series in 2012. It is known as Nakaimo for short.

Plot
Abiding by his father's will, Shougo was transferred into an academy where many young girls attend. It appears as though his younger sister (separated from him at birth) also attends this academy, and that she is trying to get closer to him without letting know him who she actually is. Problem is, Shougo seems to be popular with the other girls as well! Can Shougo "properly" reunite with his little sister and while making a girlfriend at the same time?!

Official

 * Official Anime
 * Official Light Novel

Other Sources

 * Wikipedia
 * AnimeNewsNetwork