Kono Naka Ni Hitori, Imouto Ga Iru!
Other titles :
この中に1人、妹がいる！, 三人行必有我妹, 其中一个是妹妹, 其中1个是妹妹, 3人行必有我妹, My Little Sister is Among Them!, NakaImo, IMOUTO", ?
Author(s) :Takuchi Hajime
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?!