This brings a lot of things into question. I love that Sei is finally speaking his truth and that the investigators were able to cloak that something Seiko did things to him. But it kinda makes me wonder more about Seiko's isolation. Sei sounded a lot like her while recounting his memories. I mean of course he does, she literally groomed him to be tethered to her, but how he described being separated from the family. Sei makes it sound like he's always being left out, but when you look at it from the family's perspective they're always making an effort to include him. Since he's so socially inept he had a hard time communicating and being out going with others. Aside from his relationship with his parents. Every other relationship in his life, it's always the other person making the bulk of the effort, so it's not that he gets treated differently but that he acts different.
It makes you wonder maybe Seiko was the same way when she first married into the family. All we know about her life before was that she was an unwanted child. And she was always quiet around others, but would apologize a lot. So it wouldn't be surprising that she too was socially inept like Sei. And had a hard time connecting with them. Her timidness perchance clashed with the family's out going nature. It wouldn't be surprising that she too like Sei took a lot of well meaning advances as personal attacks, and ended up isolating herself from unintentionally. I still do believe that the fam was inattentive and neglectful, and it contributed to her unhappiness. But I'm also starting to realize that Seiko might of been the cause of the isolation she was subjugated to.