My Friend's Little Sister Has It In for Me! Volume 4 by mikawaghost

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 choose to do with it? With the Alliance being pulled in all directions, will it even last the summer, or will it sink down into the depths of the ocean?