Boku wa, Yurina Onee-chan wo Ouenshite Imasui'll Cheer on my Yuri Onee-Chan; Boku wa, Yuri na Onee-chan wo Ouen Shiteimasu; I'll Cheer On My Lesbian Sister; ぼくは、百合なおねえちゃんを応援しています

Mio cares about his older sister quite a bit as she's quite shy and can't do much without him. When a transfer student joins her class, he can't help but try to help her make a friend.