Kono Naka ni Hitori, Imouto ga Iru!

is a Japanese light novel series by Hajime Taguchi. It was adapted into an anime television series by Studio Gokumi. The anime was aired between July 6 and September 28, 2012. The anime series is licensed by Sentai Filmworks and is streaming on Crunchyroll and Anime Network.

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