I never tried this before but I did find out what the "Story Mode" option does. It lets all inactive sims age and continue to progress in whatever they are doing. I think I read this for the PC version but if you turn "Story Mode" off then inactive sims will apparently not move out of town. I really don't know if this will work on the console version. I haven't tried it since I don't have a family that I want to seperate just yet.
What you can do is make a new save file and then go to 'Options' by pressing the start button and then disable Story Mode.
Disabling the 'Age Progression' option stops your active sims (doesn't effect inactive sims) from growing. With aging on, you can extend there sim life as well by using the slider.
If it doesn't work then you have your final answer but it doesn't hurt to try it out.
Inactive = Sims you can't control (NPCs, Neighbors, etc) < should include seperated family sims but not sure
Active = Sims you can control (Current Family)