18+ Watashi ga Oneechan Nan Dakara ne!

Takatsuki Misora is a 15 year old high school freshman with one glaring problem: her height. She is extremely short and is often mistaken for an elementary student and her friends jokingly tease her for being so cute. Her self consciousness is further complicated because her younger sister, Kotori who is only 11 years old looks like she should be in high school or even older. Kotori loves her big sister and wants to play with her but Misora, as the older sister, is embarrassed with what looks like a reversal of roles in a sibling relationship.

(Source: MangaHelpers)