I addressed to several points you brought up here in the comment you replied to, but you ignore them. For instance:
I pointed out the sister's reply to what happened was above an overreaction and it is not justified by the build-up the author brought. The author had to make the sister behave like that to reverse who the "victim" was and another thing she did was not have the brother acknowledge his own fault, because by ignoring that it makes the reader think losing a child that was put in his care isn't a big deal if the child ends up okay, it isn't and people fell for that.
Same as the previous one, a responsible person acknowledge their mistakes, he didn't. He is portrayed as the ultimate big brother, when his sister goes missing with his nephew after something he did, the author purposefully have Miyako to be the one to pursue her because the responsible man is pouting after a problem he created.
Once again, the same as the first point. The author had to make the sister overreact a step further because, as you also said, the sister was right, so the author had to create a narrative to change who the victim of the offense was.
I am not ignoring how he is hurt, I am ignoring how the events played out because him being hurt was born out of a bad writing moment. I keep repeating this because so far in your reply you didn't acknowledge how the sister's reaction is weird. It doesn't matter if she was stressed out for being pregnant with another man's child, or how her marriage is terrible, or how her brother lost her child and didn't acknowledge he did something wrong, when she replied to him saying those hurtful things, that wasn't a natural reaction, it was too over the top. I am not saying no one would never reply like that, but from what we were shown of her, that reaction doesn't match her character and that is bad writing because, and I repeat myself, the author had to make the brother the victim of the situation and then make Miyako a peacemaker.
The mistake he made isn't something to be understood "eventually". Isn't he such a good son and brother? He needs time to understand how delegating a child to a teenager your sister doesn't know in a crowded place this teenager has never been before is irresponsible?
Those last two paragraphs I don't have much to reply because at that point the manga already jumped the shark. Is it manipulative? Yes, but the whole situation is only possible because it was born out of bad writing and plot armour towards Miyako. I don't mind plot armour, I do mind bad writing, having a repetitive trope in a story is understandable, the way we get there is important and the way the writer wrote that wasn't good.
Anyway, most of the things I said here I had already addressed in the comment that you replied to, but you ignored some of those. You didn't address how the brother doesn't recognises his fault when his nephew goes missing (Why didn't he acknowledge it? Was he right not acknowledging it? What does the lack of acknowledgement unfold?), nor the sister's overreaction, nor how the brother was forcefully turned into the victim of this situation. I could have read this manga with a "fuck it" approach and don't put much thought on it, I didn't, maybe that was my mistake.
