My headcanon is something like that. As we've established now that Alicorns are made, not born, I'm thinking that Celstia and Luna would both have had to do something to ascend. Maybe they were amongst the Unicorn priesthood who raised the sun and moon in the old days and were the only survivors of that group after Discord's reign of madness. They had to find a way to restore the natural cycles on their own and their success had an... unexpected side-effect.
I want to know why she's white and Luna's dark. I mean, if you're the SUN princess, you should have a steady TAN, so like, make Tia BROWN or something. And Luna, who only works by MOONLIGHT should be like GHOST white. -_-
Then again, when you're the living embodiment of a god, I guess normal things like logic and UV rays don't effect you