You're banned because I couldn't actually find where I originally read that to confirm whether or not it WAS Lauren Faust. However, I did find a (different) quote from Lauren Faust stating that it was some dark magic that turned her into Nightmare Moon, which, while it isn't an evil spirit, it's close enough, really.
EDIT:Also, late reply but it was how we got onto this subject:
QUOTE(blackbishop89 @ Oct 4 2011, 12:04 AM)

Really? I thought they were fraternal twins. Then again Celestia is twice her size.
I don't believe it was ever said that they were twins. I always just assumed they were born at different times, which would explain why Luna isn't as big as Celestia. The quote I found saying that Luna was turned into Nightmare Moon by dark magic also stated that Luna isn't fully grown yet, so it definitely isn't just a case of (extreme) variation in their size, however it's still possible that Luna stopped aging when she became Nightmare Moon. I doubt it, though.
Edited by Banjo7J, 04 October 2011 - 11:18 AM.