The tidbit about Natsuko's child potentially not being human is important. I think it is possible the entire clan is cursed/blessed with some form of immortality such as the ability for constant reincarnation to new bodies, hence the requirement to kill themselves once they felt they have gotten enough out of life. Perhaps they are assisted by a mountain god in this process, but no witnesses are allowed to see this sacred ritual or the witness and his descendants will be given another curse: they can randomly be reborn as chunks of flesh like the OG rulebreaker, who himself is still around, unable to die (hence all the moans in the basement) and thus unable to reincarnate to a new body - he can only get stitched back together to then get turned into burgers again. It is even possible that every few births in the family will result in a burger baby who hosts a part of the ancestor's soul and thus also has to be kept in the basement. However as long as they keep appeasing the OG Suicide Man, the mountain god will give them prosperity.
Notice as well that when Sekiguchi was giving one final phone call to You, it is subtly implied that the family's mansion is destroying itself. This might tie in with how Sekiguchi who isn't even blood related also gets ruined/killed. Since the family's wealth is dependent on the mountain god's approval and is most likely magical in nature, he can also take everything away including buildings and people who aren't even family members when he gets pissed off enough at the family. Natsuki might have been chased around by the Suicide Man because she wasn't living in the village. Typing that also made me think that whatever the conditions of appeasement are laid down by the god, one of the most important is that the villagers are not allowed to leave the region. This might also be related to why only outsiders can objectively look at the Ancestor's current condition.