So that option you can turn on/off in the settings that is supposed to fill empty houses in your town with families... Does it actually work for people? Because I always have mine turned on because I'd like the empty houses in my town to have sims living in them without me having to do it myself but... It doesn't do anything.
Sorry I cant give you the exact answer but I can say MC Command Center by deaderpool is super effective in these cases with various settings for that down to the percentage of gender/age/occult you want to fill the lots with and also gives you notification of who is moving in or out. https://deaderpoolmc.tumblr.com/
The option always worked for me, to the point where I thought that one of my families was premade and it took me weeks to realize that they weren't, by seeing let's play and whatnot and seeing that their lot was originally empty. I also discovered they had one of the most stupid family combo possible: single mom with kid and teen AND another random adult single woman living with them, with absolutely no relationships nor family ties with any of them. Oddly enough it turned out to be one of the most interesting 'families'.