お姉ちゃんの3乗（きゅーぶ）

College student Arisaka Miku lives alone with his cousin two years his senior, Shunzaki Nanatsu. The pair spend their school days as any would until one day, a mysterious distortion in the space-time continuum splits Nanami into three girls. Their names, looks, and personalities are all different but they are certainly still "Miku's older sister". Then a fairy from another world comes to investigate the distortion...

So begins this strange tale of cohabitation with three times the onee-sans and three times the pampering!