I admit I read quite quickly and skimmed some parts but didn't it say he was found dead having wondered into the woods because of dementia?
if he was murdered by the husband then idk why the husband wouldn't admit it since he was already admiting to killing the other guy.
If Yuzuki was really trying to get revenge on shigeharu then surely there's a bunch of other ways that don;'t involve tricking your own husband into murdering a child and forcing your own daughter to raise a a child to commit murder? like she could have burnt the house down while shigeharu slept or poisoned him while making meals for him or a bunch of other things.
crazy.
Also, what's the point in going to all those lengths to avoid having Touya commit murder when you're still forcing touya to live in a single fucking room with no windows for 4 years of his life???
That's still outrageous child abuse done just because your wife is scared of her rich grandfather. Seriously what the hell?
His plan should have just been "as soon as we get married and become guardians of Touya, we take him to a local journalist who helps write a story and then go to a child psychologist who does an evaluation on Touya to confirm that severe abuse has taken place because Touya has been raised in a single room his whole life and is on no official register and has never gone to school then go to the police."
Bish bash bosh. Seriously what is rich grandpa going to do? He isn't a mafia don. He's one eccentric loony with a sycophantic nephew.
All right, so first, European culture was not at all different, when it still had aristocrats, there are books with similarly fucked up stories basically up until WW2. I remember reading book about some US couple go to Itallian countryside soon after war and get embroiled in bloody and cruel family story of local counts, where most stuff happened before war, when no one in the village/small town would have thought to oppose a local noble. That was all fiction, but so is this manga.
In modern days you can have rich people have underlings get them drugs and underage prostitutes, and in yakuza or mafia families - or cults - you can get somewhat similar "you can't leave the family, and you can't oppose evil stuff in family", but aristocracy adds the cult-like loyalty to "rich and powerful family" aspect, retainers serving them for generations, and allows for some fucked up family codes to persist because noble kids are normally raised believing in their duty to uphold family traditions anyyway. US didn't have aristocracy, and Europe doesn't really either since WW2, but I guess in some Asian countries the ethos of noble clan stayed for longer. And to be fair, it's still pictured as really weird and anachronistic in this story, it was supposed to be something of a bygone era.
As for Ayano and Keita, I can at least understand them being scared to go against family, if grandpa could already order both kidnapping of Touya, and take Ayano from her parents, with aunt Misaki just disappearing somehow. If mother and aunt couldn't oppose grandpa, they could believe it's just impossible. They could also risk losing Touya, since they weren't his real parents, and with whole family proved to be criminals Touya could be taken to orphanage. Since they only had to fake four murders, they could count on old guy dying and then getting free of this nonsense, without getting , very negative in Japan, stigma of being part of criminal family.
The real big plot hole is with both Yoshi and Misaki, especially Yoshi; normal adult women from normal families they could turn to support for, letting their kids be taken like that. They could stop the plan any time by working together and turning to police, they could even leak the plan to Soichiros and ask them to confront grandpa, which due to ridiculous requirements of ritual murders would make them impossible to carry out, sparing everyone the drama. And why didn't Misaki's family report anything about her to police?
Author tried to plug that hole with suggestion that Yoshi planned this and undermined Misaki, but this is even dumber version, and I agree with your criticism of that part. Why would she destroy her own family for revenge for her grandma's sibling? How would she know her older daughter wouldn't break and become murderer, doing grandpa's will? What would she plan to do if there wouldn't be any arm-less kid, and why didn't she use that plan instead of condemning her daughter to hell of being raised as future murderer and child abuser? Nothing about Yoshi or Misako makes sense. Mother could just stop the crazy ritual murder plan, and see grandpa go (more) crazy and die from heart attack, afraid of revenge of the ghosts.