kono naka ni hitori imouto ga iru!
この中に1人、妹がいる!, lit. One of Them Is My Younger Sister!, abbreviated as NakaImo (中妹, なかいも)
A 2010-2013 light novel series by Taguchi Hajime, with illustrations by CUTEG, published by Media Factory's MF Bunko J imprint. Also adapted into 2011-current manga, Kono Naka ni Hitori, Imouto ga Iru! - Who is "IMOUTO"? by Mottun*, serialized in in Media Factory's Monthly Comic Alive, and a Summer 2012 anime by Studio Gokumi.