"This is a story about 12 sisters who love their older brother. They have loved their brother since they were little and were happy just being together,

however, due to certain circumstances, they have to live apart from each other... They only get to see their brother once every two months, and they cherish the time they get to see him.

Each volume focuses on a different sister and is a separate story. Those books were released in batches during "second season" of Sister Princess project serialization in Dengeki G's Magazine."

(Source: Bookwalker)