So just some background things that might help someone figure out what is going in
- kodoku (poison jar) is an old Japanese folklore curse technique where you start off with a jar of insects and as they eat eachother, you use the last remaining insect as the curse medium. In manga, it's used as a last man standing/death game/the strong eating the weak metaphor/symbolism.
- the ritual required the use of 4 doors and 4 deaths. So the number 4 was important here.
Just a side thing but I think parents in the photo are likely the same as the hiring couple. They look similar but older.
I think the mother in the photo was actually being interviewed on TV after the body of the daughter was found, that's how Yagi knew the two people who initially hired Chacha were fakes. And the woman on TV looks exactly like the one in the family photo.
Anyway, I read the arc again and I noticed that the posters with the woman's face while they were searching the house had the same name as the name mentioned as the "reincarnation" at the end. So it does seem that there was some kind of cult where a woman was a cult leader who was proclaiming that the end of the world was near or something like that, but she eventually died, and so two members of the cult lured a number of detectives to this house to complete some sort of ritual to allow their cult leader to be reborn.
The "poison jar" thing seems to have been connected to why there were two children's desks in the house but only one child in the family photo. Apparently one kid was bullied by the other kid, saying that they were stupid and that their mother liked them the best, and so on. Which could have triggered a sort of "battle" between them, hence the poison jar analogy, and the stronger one became the vessel for the cult leader's reincarnation.
What I'm wondering is if the initial missing girl was even connected to this whole thing? Like if she was a daughter of this family, she would be the one who was "fighting" her sibling in the poison jar, but her body was found elsewhere as said by the TV news? But at the end of the chapter there's clearly a hanging body behind the reincarnated cult leader in the son's body, which would mean that the hanging body was the actual sibling. Did they have three kids or something? Or is the missing girl totally unrelated?
Edit: I also noticed that the kid at the end had a different hair style than the kid in the photo, perhaps implying that this was the kid being bullied by their sibling, but he eventually "won" the war of the poison jar.