Wow ... the author really does explore every nook and cranny of the heart doesn't he? The mother is obviously super conflicted, this is someone else's boy who came into the family. That alone is enough reason to make it hard to love that person like your own, especially when it wasn't your choice to bring him in. To top that off, the husband ended up neglecting his own kids and focused on this foster kid instead. The husband related more with the kid than anyone else, and understanding that reality in itself must be painful for the mother. On top of that, she can't even fault the kid. Rei is good natured at heart, and was literally the ideal child while he was with them. And yet she could not help but distance herself from him because of all these conflicted feelings involving her own kids in comparison to Kiriyama. It seems like she understands she's not acting adult-like, nor being very mature about this matter, but she feels helpless regardless as someone who also didn't have the kind of drive her husband or Kiriyama showed. Almost like the teacher who broke down mentally, the mother as well felt too helpless to do anything about it and instead let the more immature side of her heart take over.
But at the core, she also seems like she also feels something very off about Kiriyama as he's somehow acting way too mature for his own age. Her seeing Kiriyama in her dreams, acting like a normal kid would appropriately for his age, gave her a sense of relief. To be able to see him just be a kid. And in this ending there is also a subtle lament for the childhood that Kiriyama should have had, but lost due to his predicament. Man fuck this author brings out emotions from the depth,