Mother Celestia and the Luna have no name, therefore I will simply call her "She"
She was born from some distant star and is a powerful creature whose motives we will never know. She is not God and She didn't create Earth and ponies. She just happened to find this planet and decided to stay on it for a while. Who knows why… She did not create anything on this planet, but her presence affected everything. Her appearance got the most reasonable creatures, ponies. The only ones She created are her daughters, Luna and Celestia.
At first, She and Celestia lived quietly somewhere far away. Celestia learned to control her abilities… Until they were found by a young and powerful unicorn-leader of the united tribes-Starswirl. He sought salvation because the magic of unicorns weakened with each passing decade. One day there will be no one to raise the sun. And he found their, real gods! Beautiful and powerful!!! By that time, She felt that she would soon have to leave. Therefore, She did not show herself to ponies. But she stayed with her daughter, invisibly. She supported her when she raised the sun and began to rule the Equestria with Starswirl. And at some point she just left. Has dissolved and whispered her daughter that one day they will meet again. She disappeared, leaving behind a tiny foal, Luna……
After the departure of a strong and bright creature, the planet was attacked by darkness: chaos, ponies of shadows, sirens and many others …. Starswirl sacrificed himself to stop one of them. So our princesses were the only rulers of Equestria.