Futakaku Kankei.  

Bicornuate Relationship, 双角カンケイ。

The two sisters, Airi and Himari, have always been very close and shared many things together, including their hairstyles, clothing, and accessories. However, as they entered high school, Himari began to distance herself from her sister. When confronted by Airi about this, Himari revealed that she had made a promise with someone important, known as "senpai," which she felt obligated to fulfill, regardless of the consequences.