A Sister's All You Need.

Synopsis

Itsuki is nearing his final deadline for volume 5 and flees from his editor, Toki, however he is immediately caught. However, Itsuki manages to escape when he runs into Setsuna and they both go to a hot springs. Setsuna felt like he was in a slump and wanted to see some asses to get some inspiration. But when they get to the ryokan with the hot springs, Itsuki witnesses something horrifying...