Well I was a citizen in my brothers town in AC, eventually I got a mannequin from Sable, then I got my own copy of the game and moved to a new town. I kept all of my belongings, including the mannequin. I know that you can only get four in the game, but since I basically started a new game is it possible that I could get another mannequin from Sable?
I started my character in a friend's game before getting my own. I got my first mannequin today in my own game after moving about 2 weeks ago. The trigger seems to nothing more than how many items you buy from the sister's shop. However, I never received a mannequin while living in my friend's town, but I don't think that would have any effect.