My Friend's Little Sister Has It In For Me! Volume 4 (Light Novel)

Mashiro has just shared a devastating secret with Sumire. Aki and Iroha have been forced through a ceremony that historically guarantees the participants will be married. Meanwhile, Sumire is on the brink of freedom—but just what will she do with it? So much for Aki’s efficient summer plans! Worse, he finds out the foundations beneath his Alliance are pillars of salt and sand. With the Alliance pulled in all directions, will it even last the summer, or will it sink into the depths of the ocean?