This was a great ending. Iroha wanted to die, just being alive is an inconvenience to onii-chan and makes him hate her, so she decided to stop being alive. But instead of dying, she had brain damage. Because of it, he does not hate her anymore. Because of it, He doesn't see her as a nuisance anymore whenever she does what he does, instead he was the one lovingly teaching her things at the end. Because of it, she has been born all over again to be a cuter little sister. Because of it, onii-chan no longer gets mad with the idea of Iroha writing a novel, and was, just as Iroha wished originally, writing stories together with her, and even welcomes Iroha wanting to write a story with him in the end. Iroha indeed did not actually change that much on the inside. She was doing everything for onii-chan before, she still does now. Onii-chan is the one that changed. Instead of being mad or jealous, he now feels love for everything Iroha does. "My little sister really is a genius". Words Onii-chan uttered in the past full of disdain, jealousy, and hatred, are now uttered with acceptance, affection, and love. In the end, she and onii-chan found a way to be happy together.
Iroha wanted to die, but instead, she got a second chance, a chance to be happy with the one she loves the most in the world once again. This was a happy ending, one that gave me enough post-hentai manga clarity to write my tears away. 10/10