Soukaku Kankei.

is a japanese manga series written by Tachi. It began serialisation in the March 2015 issue of Manga Time Kirara Forward.

The series was released under 2 volumes from March 2016 to January 2017.

Synopsis
Twin sisters Airi and Himari have always been very close, and worn identical hairstyles, clothing and accessories. But after they enter high school, Himari starts to drift away from her sister. When cornered by Airi, she explains that she has "a promise with senpai to fulfill," no matter what.

(Source: MU)