My Friend's Little Sister Has It In for Me! Volume 5 by mikawaghost

The Alliance is caught in a maelstrom of trouble as secrets are covered and uncovered everywhere you look! Unaware of this spiral of secrecy, Akiteru’s started seeing Iroha in a new light... And just his luck, his uncle shows up to make sure Akiteru sticks to playing the role of Mashiro’s fake boyfriend!

Akiteru’s got no choice but to set up a (fake) date for the summer festival. Will someone like Iroha really sit back and just watch? What’ll become of the Alliance in this perfect storm of trouble? And...just who will Akiteru wind up watching the fireworks with?