My Childhood friend calls me Onee-Chan

The life of Yui Tsukomo and her childhood friend Maketa Sato who calls her Onee-chan even though Maketa is older than her by few months. The reason for this started at a New Year's eve many years ago when both were seven years old. After all that embarrassing moments due to that single word,Yui Tsukomo promises to herself to stop Maketa from calling her Onee-chan which leads to a life which she never imagined.