Well, I'm sorry Ginger! It's just what I do, it's sort of literally my job to come up with shit like this.
I also have another headcanon established for
The Colour You Bleed - Discord is a primordial god, one of the first and true divinities in the world of MLP. There are other primordial gods as well, but most of them have phased out. They had control over concepts rather than things, so there were also gods of order, creation, destruction, etc.
Luna and Celestia aren't true gods, but rather old souls. Old souls are the spirits of dead ponies who accumulated astounding amounts of magical power in their lifetime, enough to create echoes of themselves in death. The echos latched on the the ambient magic in the world, and started to bond with it, essentially becoming creatures of pure magic. In the old days of primordial ponies there were hundreds of old souls, as their magical power was much more raw and massive. These days, only someone like twilight could become a old soul, and only through pretty intense training to buff up what she's already got.
The old souls could become gods when they were born, either through ponies or through bonding themselves to certain objects or elements. When they do, they become gods over them, because they are literally part of it. That's how you get gods like the god of theshifting sands, or the god of a forest. More powerful old souls could bond themselves to big things, like the sun, moon, and ocean. Luna an Celestia didn't bond themselves to the sun and moon originally, but were actually the product of three old souls combining themselves:
the three kings
. They took control of the sun and the moon when they got older and more powerful.
There's more to it; old souls could make allegiances and exchange power among themselves (that's how Celestia took control of the moon from Luna), so you could have one astoundingly powerful old sould because a whole bunch of others swore fealty to him. But it gets pretty deep, and essentially unimportant to the 'how Luna and Celestia came to be' aspect of the headcannon, and if I wanted to talk about all of it I should probably take it to the fanfic thread.