If people don't understand how poorly written this is, either they are too young to understand how the sister is RIGHT as a mother and how the author put herself between a rock and a hard place to solve this matter and did it in one of the worst ways possible, or people have shallow text interpretation and ignored the sister as a "hysterical" person and just focused on how Miyako is so smart to have devise this plan when it is quite literally plot armour.
I still don't get your objection. Of course the sister was right. Like, that's the whole point. She was justifiably angry, but said something in the moment that was hurtful to her brother. Instead of "you stupid moron she's a teenager" or "come the fuck on, you know better than this," she hit him with the "you don't have kids of your own." For him, someone who wanted a family but hasn't ever had the chance to have one... that hurt.
I don't wanna pull the 'you obviously need some life experience' card but since you already did, the dad here has had an awful lot of characterization the whole time. As the main character, of course he has. But his characterization is that he's responsible, lonely, and wanted a family but consistently made decisions that prevented that. Including marrying who he did; an older woman who can't have children. There is a real, deep sadness in him about never being able to have a child of his own, and its a sadness that is super recognizable to me. It's something a lot of older, professional, men feel, people who lift their heads up and realize that they've missed their shot. Having to give up a dream hurts, and his sister hit him hard in a very hurtful place.
I feel like you're missing this part. Understandably, because you are focusing on how the sister was wronged and how her actions were justified. But you're missing that he was actually hurt and why he was. Without this, the whole scenario makes no sense.
He
could have come to understand that her anger was justified, eventually. He got into his own head about how he was just trying to be a good son and brother and how his family was always on his ass, but that was just one thought. But the mom came in that night and said that he didn't need to care about anyone else anymore, because he has his own family now. This was the obvious outcome of this plan, driving him away from his family.
The second part, that the
sister was a target too, is the unexpected but natural outcome. Cause the mom is apologizing here, saying that the sister was totally in the right, that she doesn't have any reason to feel guilty, that if she just talked to her brother everything would be fine. That it's not a big deal and that the sister's reaction is both understandable and correct. This is the empathy that the sister desperately wants and has not gotten; someone to get why she was angry and to tell her she was right to be so. No one else has done that yet.
Rather than making the sister out to be a hysterical bitch, the mom's plan was much more insidious: set up a situation to get her to say something that can hurt her brother, whisk the brother away and foster his resentment, then reach out and tell the sister that her reaction was totally justified. This scenario does not depend on thinking the sister went over the line. Instead, it depends on the sister being
totally justified in her reaction, so that when the mother comes, apologizes, and says that there's no reason for the sister to worry about it, it's easy to believe. Which will lead to the sister opening up about her pregnancy, something the mother/daughter pair somehow are already aware of.