@snip36 One thing really neat about this series is that it's really hard to point to any one moment as the moment where Mahiro fully became a girl. It could've been as early as chapter 3 or 4 of the first volume but it's otherwise the transition has been pretty gradual. Pretty safe to say no matter what, Oniichan has been done for a long while now.
IMO the moment of no return was vol 2 chapter 18 part 2. It's where Mahiro is very visibly distressed about the idea of going back to being a guy and and there's even a partial manga title drop. What follows is a scene where Mahiro actively chooses to not lose the chance to stay a girl even when Mihari offered to let him go back. That's when Mahiro took part ownership of their condition, even if there were extenuating circumstances that made it the less stressful choice.
(still though, as I said, there's no single watershed moment you can point to so the above is just my interpretation. Honestly it's kind of refreshing because this is how identity realistically shifts in my experience)