can I get demoted from manager to sales clerk without ever being written up so my supervisor can promote her daughter???

Though unfair, the manager can prefer his daughter over you and then demote you without violating the law.  If, however, there is a corporate office or the higher up people are not aware of this, they might review the matter IF they are the type that would believe it as not appropriate for the daughter to be promoted.  Otherwise, there would be nothing else you could do