The details are sparse (makes sense - it's an outsider POV) but it's fairly straightforward:
Long ago, a taboo was broken, and now, generations later, the family still has to deal with the consequences of it. Yousuke, the steady boyfriend, just happened to be someone who is invested in being in the good graces of the family and is trusted by them while still not being completely part of them. They used to him check on the status of whatever appeasement rites the previous generation did.
The scattered skeleton was a Bad Omen, so the family and Natsuko basically wiped their social life to prevent the curse/vengeful entity from catching anyone else they might treat as "family". It got to the servant after all, so the it acknowledges non-blood relations as well. The house being in ruins when Yousuke goes to visit again could have been the result of the curse getting aggressive, or just the family further isolating themselves to prevent the curse from spreading to other people.
The dream is likely Natsuko visiting Yousuke from the afterlife. Either way, Yousuke saw the skeleton whole when Natsuko left, probably meaning the curse doesn't consider him part of the family anymore. And he wakes up with a sad closure about Natsuko's disappearance.
There might be creepypasta or folklore context we're missing but it's still understandable without it!