That's your own personal headcanon. There has never been anything stated in the show about Celestia and Luna's mother, much less her being named "Galaxy". Faust technically is Celestia's and and Luna's mother since she created the characters.
The fact that nothing about Celestia's and Luna's parents has been officially stated by the actual creators of the show means that while I'm sure you like the idea of their mom being a different alicorn named Galaxy, it is not official, nor is it true.
If you saw something about that, it was most likely a fan's own personal theory, no one knows for sure who Celestia's and Luna canon parents actually are. I hope that might have given you a bit more information on what is actually true regarding Celestia's and Luna's parents.