Duke of Canterlot

Princesses and Queens of the past like Princess  Amore, Queen Parabola, etc

I think princess Amore is a distant cousin of Celestia and Luna , since their family tree clearly has unicorn somewhere in it because prince Blueblood is Celestia's distant nephew 

Also Princess Amore is a white tall unicorn that bares a similar look to Celestia,  and I know there are other ponies that fit this mold but, Celestia would have known about the crystal empire and have the atorthy to put Cadence in charge only if she was related to the former ruler ,since it is a separate empire to what Celestia rules but still considered part of equestria. 

As for Parabola let me know your theories