Not gonna lie, when I saw the "ahahaha haha" panel at the end I was thinking "Oh no he wouldn't would he?!" but she was just feeding him (lol I've read too many incest doujins I guess)...
Yeah the dad was irresponsible from the start and did see the problem since the beginning.
He's the type that shirk out of any responsibilities and the type that will let his wife be NTR
I think the boy did it. He clearly had a flashback when his cousin fell from his hand in the house. He probably begged the aunty or something to help back then
Husband has no balls; a shame. Husband was so happy to get such a young, pretty wife; he had no idea just how nuts she is, and has no real mechanism for dealing with it. Where is Seiko's family, I wonder? I don't remember them being mentioned ever.