Ok so, this theory has been in my mind for a while, I don’t know if it would work but what if Principal Celestia was actually Sunset’s birth mom? And that Sunset accidentally fell through the portal when she was little and was raised by Princess Celestia. What if Principal Celestia
put Sunset up for adoption when she was born and Sunset ran away from the orphanage. Maybe that’s why we haven’t see the other Sunset Shimmer because she was actually born human but fell through the portal.

The first sounds like the best idea

I know there is at least one story that does something very much like this. The biggest difference being Principal Celestia isn't Sunset birth mother, but she does adopt her.

7057152
Could you send me the link please? :)

7057155
I’m surprised if you’ve not read it already. The adoption occurs in previous story (this is a sequel), but Sunset origins are revealed in this one.
The story covers a lot more than just her origins. Apologies if it’s not what you’re looking for.

https://www.fimfiction.net/story/348740/making-family