Who are Princess Celestia and Princess Luna's parents?
Unknown and (as yet) unstated. Her parentage hasn't been revealed for fear that it would de-rank her.
"I know "princess" sounds cuter and helps sell more toys, and because of that you were probably forced to keep her as a princess by the powers that be..."
LF: That's what happened. I was told that because of Disney movies, girls assume that Queens are evil (although I only remember 1 evil queen) and Princesses are good. I was also told that the perceived youth of a Princess is preferable to consumers.
She does not have parents that outrank her. I brought the weirdness of that situation to my bosses, but it did not seem to be a continuity concern to them, so I'm letting it alone. I always wanted her to be the highest authority, and so she remains so. And I certainly don't want marriage to be what would escalate her. (Bad messages to girls and what not.)
Sorry I couldn't give you a more satisfying answer. Maybe as the series goes on we can thread something together. I put up a bit of a fight when her title changed, but you win some, you lose some.