From all the goings on this volume, the patterns under the cover of volume 2, the ending of GLT and some other things I'm thinking this entire series is a sequel to GLT.
No bodies at the end of GLT and the crumbled wall with the strange pattern combined with Chito and Yuuri being here make me think they got sucked into some simulation of reality at the end of GLT. All the weird stuff going on is the simulation messing up, the "gardener" is meant to rectify these errors to maintain the simulation, and the sister is trying to break out because she knows something is wrong/for undisclosed motives.
Or tkmz just wanted to wax philosophical for a few volumes and brought Chito and Yuuri in to draw one of those "parallel universes" mentioned in the Anthology.