Look...I'm serious here...each save - each instance you make...each time you enter Create-A-Sim and make a new family and install them in a neighborhood the WHOLE F'ING neighborhood resets. All the pre-made sims don't know any of the sims you previously made and installed in that neighborhood and the new family you just put there cannot possibly meet the previously installed families you put there because they are in their own Sim-universe which was started the moment you put them there. Sliders and Groundhog Day. Seriously.
I wish people would stop saying this. It is absolutely possible to play a family, then decide to create a new family, and have them exist in the same neighborhood as your first family. Just go into Edit Town and go into CAS from there. Get out of CAS, place them in a house, and they are in your current, active neighborhood. You can then decide to switch to them as your active household, or go back to the first family. Either way, both are now in the same universe.
Multiple saves files are meant only for creating multiple copies of the neighborhood. Don't want parallel universes? Don't create more than one save.