This story made me legitimately uncomfortable. No not because of the kid that was just silly, but because if I had had to do this when I was a small child, I would have been traumatized.
Why would this traumatize you? Getting naked around your family members should be normal, it helps you get more comfortable around them.
But see the author's note: he says he was comfortable around his family, but when he went on the men's side with three relatives as a youngster and saw his classmates he was very uncomfortable.
So in a way he agrees with the commenter. above. Nonetheless on balance he says it was a good experience. I share that opinion though i was in my 20s when I started occasionally using public baths for various reasons in Japan.
On Edit: I thought it might be a pseudonym but Baka-updates, which is usually very good on this stuff, says Sakurai Makoto is, indeed, a male.
I suppose that makes sense. It's a bit hypocritical of me to say that too, since I'm too shy to even change around other women.
I can imagine that since Cotarou is so used to being around girls all the time and even comfortable enough to be naked around them, he'll probably have an easier time with the ladies when he gets older. Or maybe Nico ended up rubbing off all her gayness to her siblings, who knows.