My Friend's Little Sister Has It In For Me! Volume 8 by tomari

It's time for the biggest event of any high school career: the class trip!With Iroha out of the way, Mashiro's more determined than ever to deepen her (fake) relationship with Akiteru. Sucks for Mashiro, though-a third girl is now vying for Akiteru's affection! And Iroha's launched into a long-distance offensive to pester him, but will that be enough for her?With so much trouble brewing, this "relaxing" trip to the hot springs will be anything but!
Binding: Paperback / softback