So... chapter 15 is... a lot.
The explanation of the metaphysics is messy, even confusing, though I understand enough to follow where it intersects with the main cut and thrust of the narrative.
The reveal of Yuuka having a different but complementary power works fairly well, and explains why she keeps Ayaka so close, it also fleshes out more of how Ayaka's ability functions, even if that... was the bit that kind of lost me, revealing she has untapped capability.
The chapter also has an impressively grim final page, with existential dread a-plenty... and yet all of this just keeps painting Yuuka as Concerning at best and Predatory at worst, to say nothing of the tragedy porn of the accident itself.