@ImRaidAccount  

 
We know from the Twilight chapter book that Cadence was found in the woods by a village of earth ponies and raised as their own before Celestia adopted her. But she could still be Sombra’s great-great-great-great-granddaughter, descended from the child in this theory.

@ImRaidAccount  
Except the Cadance book that directly disproves it quite a while before the comic ever happened.
 
And ya know, being only a teenager when Twi was a kid in show canon.
 
Sorry OP, but your headcanon was DOA.

This doesn’t account for how Cadence lived that long but a relatively sound theory.