In the Beginning (Patreon)

concept art, celestia and luna pre-alicornification

Celestia was a pupil of Starswirl the Bearded (don't judge her, beards were very hip for unicorns back then), and Luna was a troubled  pegasus with the mysterious power of dreamwalking. Considering them not actually being blood sisters, but rather taking a vow of eternal sisterhood after going on an impromptu journey together to save Equestria and restore the balance of the sun and moon.