I'm looking for a book I read in the early 90's about a princess who is exiled after her family is murdered. I remember her family shared the throne with another family (her cousin I think). I believe her family had dark hair and complexion and the other family was golden or blonde.
She ends up being in a cavalry troupe for a noble or rich family but nobody knows who she is, except the younger son of that family. I think his name was Niall (maybe) and he was training to be a mage. She regains her throne at the end of the book and rules with her cousin. She also had an arranged marriage with a guy 10 or 15 years older than her, and she was able to use a little bit of magic herself.