Happened to me, too - moved my sim out of the family home into a new active household. After a few (human) hours of playing, they all showed up in his relationship panel as 'deceased', i.e. they all died of old age (including his younger sister) & the neighbourhood was completely empty.
I started a new game, and turned off aging for all except the active household in the Options menu & it seems to have fixed the problem... hopefully that works for you!
All died?? Weird. I just had the fiancé and father of one of my sims' children vanish from the face of earth after my sim left his house/area. Last thing I saw of him was him running around in order to leave the area. Just like that. He didn't even die. He was simply gone... But his household was still living next door... with no one in it...
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I started a new game, and turned off aging for all except the active household in the Options menu & it seems to have fixed the problem... hopefully that works for you!