90s sci-fi/fantasy book series with a girl who can control fire, main enemy is her husband's uncle

Question

The first book in the series begins with a girl named Scion (something like that) and she lives at a school where they are teaching her to control fire (firebending or light bending). This seems like it's way in the future, no technology like electricity/phones...

She has to leave to be married to a prince out in a desert country. She feels out of place, then they fall in love, she makes enemies and at some point the king dies so she and her husband are the king and queen.

Her natural talent allows her to create fire from sun or light and at some point, in defending herself she gets a permanent burn scar. Her main enemy is her husband's uncle and his dozen wives; they all have daughters and he wants to be king.

Also they kill dragons for sport until she and the prince figure out that they can make gold or something with their fire breath (at a dragon breeding cave).

They're all traveling to a big outdoors trading festival called Rialatta or something...

There are at least three books but I think more.

Answer 1

Sunrunners' World / Dragon Prince series by Melanie Rawn.

An arranged marriage between Prince Rohan of Stronghold and the "sunrunner" Sioned of Goddess Keep promises to bring a new kind of peace to a strife-torn land until the machinations of a self-serving High Prince plunge the world into war.

The festival is called Rialla.

From Amazon's summary:

When Rohan became the new prince of the Desert, ruler of the kingdom granted to his family for as long as the Long Sands spewed fire, he took the crown with two goals in mind. First and foremost, he sought to bring permanent peace to his world of divided princedoms. And, in a land where dragon-slaying was a proof of manhood, Rohan was the sole champion of the dragons, fighting desperately to preserve the last remaining lords of the sky and with them a secret which might be the salvation of his people....

Sioned, the Sunrunner witch who was fated by Fire to be Rohan’s bride, had mastered the magic of sunlight and moonglow, catching hints of a yet to be formed pattern which could irrevocably affect the destinies of Sunrunners and ordinary mortals alike. Yet caught in the machinations of the Lady of Goddess Keep, and of Prince Rohan and his sworn enemy, the treacherously cunning High Prince, could Sioned alter this crucial pattern to protect her lord from the menace of a war that threatened to set the land ablaze?