[@Sir Lurksalot]
I suppose the difference is that Duncan is the youngest in his isn't he? Vivi has a younger sister, though her magical prowess and divine protection smash her on all counts (Fun fact: Vivi doesn't have a divine protection :P Her strength is pretty much entirely based on hard work and determination to stay useful. A Celestia who is useless isn't a Celestia for long.). Though she is very, very antagonistic and destructive.
Hence why they keep her under lock and key XD