The words that sum up the theme of this story arc would have been chilling even if they weren't spoken by Inari: “how to make use of children.” In part that's because the concept of childhood is, historically speaking, relatively new, and today we understand what possibly-long-lived kemono Inari doesn't: children don't exist for you to “make use of.” That she sees Inugami as weak because he doesn't only take advantage of the kids under his care says a lot, and when we add to it the yuki-onna world-view about what was acceptable to do to Yui, even Inugami's sometimes careless approach to childcare seems like the kemono gold standard. Because Inari and the yuki-onna don't see childhood as any different from adulthood and act like adolescence is a myth; to them, children are just smaller people who can be used.
That that's precisely what has been happening to Yui, of course, was never in doubt. From the moment we saw the shriveled fingers of his father through the barred window of that hut, we've known what was in store for him and Akira, and it's only because Yui was able to find the strength to protect his twin brother that Akira was able to escape. Even though the episode never uses the words “rape” or “abuse,” it's evident that that's what had been happening to Yui back in the snowy village; Akira's innocent observations of his brother's late returns to their house, Yui's increasingly drawn features, and most especially the scene where a yuki-onna corners him and demands to know if she's pretty all speak volumes about what he's enduring. We don't need to see it to know the truth, and Yui's panic when he realizes that Akira has also hit sexual maturity again says almost everything we need to know.
One of the sadder things about that scene – and the whole situation, really – is that Yui sees himself as unclean or polluted (as the subtitles phrase it), while Akira remains pure. While we can certainly frame that as an emotional state – innocence versus experience – it's perhaps more a case of Yui viewing himself as no longer having any value now that he has been sexually attacked. He may see the fact that he was able to have sex with the yuki-onna as a sign that he was complicit in his own rape, therefore sullying him even if it wasn't anything he wanted. Akira, who remains virginal, is therefore somehow a purer being; his emotional innocence tells Yui that he is unspoiled. Never mind that Akira's personality was always more innocent than Yui's; to his mind, being forced to fulfill the role of a yuki-onoko would destroy one of the things he loves about his brother.
That may be true in the sense that undergoing the same sort of sexual abuse would have a damaging effect on Akira's psyche. But Yui has come to see Akira as the last good thing in the world, and he can't bear to think of anything hurting or spoiling him. That's why he can't allow Shiki, Kabane, and Inugami to take Akira away and why he's actively fighting Nobimaru: no one, he feels, can protect Akira as well as he can. Seeing Akira happy has perhaps taken the form of the only comfort Yui can imagine feeling. He may even see all of his past efforts – stealing the nullstone, probably destroying the snowy village – as having been done for Akira's sake, when in reality, he's reacting to what was done to him and saving himself from further abuse.
How long can he keep that up? Given that Inari says that the nullstone feeds on life, Yui may be chipping away at his own lifeforce every time he uses its powers. Will removing it save him? Does he even believe that he can be saved? I'd like to think so, or at least that Inugami will try. Kabane and Shiki both certainly understand the importance of family, so even if Inari hadn't said that the nullstone will help to find Kabane's parents, I think he'd give saving Yui a shot. (And please, please don't give Inari the nullstone.) Next week may be the final episode, so here's hoping things end on a hopeful, if not a happy, note.