Things I've done in the past when such situations show up...
Go into Edit Town mode and evict the family, then put them back in and make them the active family again. Now try your move.
Go into Edit Town mode and split up the household, moving out only the sim you want to move. Then pick their lot for them manually in Edit Town rather than through the move function in-game from the cell phone.
The standard, clearing out your caches in the Electronic Arts folder before starting the game, has also worked for me for a whole range of bugs including ones very similar to this.
You can also try when you reload the affected game, to do a "Save As" and give it a different name. Then exit the game, restart TS3, load the new file.
Those are the simple ones I would try first before taking any drastic measures.