Lady Sixteen

From MangaHelpers:

Twenty year old Haruka and sixteen year old Nanaka are not only sisters, but also best friends. Haruka works as the editor for the local newspaper while Nanaka is a high school student. Nanaka wants to be just like her big sis when she grows up and Haruka can’t help but bring out her childish side when she’s with Nanaka. The large age gap doesn’t prevent them from being best friends and sharing everything with each other. [vyc]