I'm glad I re-read this, because I think I may have misread it the first time. I'd come to the understanding that Hiroki's brother, in watching this, might have learned he wasn't actually her brother and that was the twist at the end, but I think you're probably more on the money with this one.
On that note, though, shout out to an old friend of mine who, when I showed off a video I found that I think was of a Korean daytime talk show segment where they take boys and give them feminine outfits and dresses to make them look like cute girls, responded to the effect of "oh, this makes me feel a certain way" and the next time we came in contact was out as a trans lady. I doubt you'll see this but I hope you're still doing well!