Celestia and Luna married the mane six, now they have a harem and a honeymoon to deal with. In all honesty, happily ever after isn't all it's cracked up to be. At least there'll be room for orgies.
A thousand years ago, Princess Celestia banished her sister to the moon, that much we know. What wasn’t known, and what Celestia kept secret was that before her banishment, Luna had given birth to a foal; Twilight Andromeda Sparkle.