According to the official website, Taguchi Hajime’s (author) and CUTEG’s (illustrator) romantic-comedy light novel series, 「Kono Naka ni Hitori, Imouto ga Iru!」 will be receiving an anime adaptation.

The NakaImo light novel series currently has 5 volumes to date – with the first volume debuting on August 31st, 2010. A manga adaptation by Mottun* has been serialized in Comic Alive magazine since May 27, 2011.

*Synopsis:

As his father’s will, Shougo was transferred to an acadamy where a lot of girls attend. “while attending an acadamy find a girl that will be your partner” in other word his father’s will was Shougo to make a girlfriend. However he found about that there is his little sister sepreated from birth and he doesn’t even know how she look like. Then in Shougo’s birthday, he recieved a cake from unknown sender, and his celephone’s ring tone rings. “Happy birthday, Onii-sama. I adore you very much.” it looks like his sister is also attends to this acadamy, and try to get closer to him without letting know him who she is. Can Shugo “properly” reunite with his little sister, and make a girlfriend!?

[Source]