1. Have her play Teenage Mutant Ninja Turtles and see how fast her wrist hurts... This is to prove her wrong about the motion controls.
2. Show her.... pretty much any game screenshot on any other next gen system, Crysis if you want to show her one of the most graphically advanced games ever made (it may not be on consoles yet but what the hell right the PC screenies will look better anyway).
3. Then remind her the other consoles (or PC) have far more top notch games not just sludge for 5 year old kids.
4. If all else fails just say you'll buy the X-box (or PS3 both are good systems) with your own money (which it sounds like you will be doing anyway) and let her pay for the Wii herself and let her find out the hard way that around 75% of Wii games will ruin your wrist.
Hope this helps