The Two in Hiding — Ru Emerson

STRANGE AND WONDROUS RUMORS
... spread through the kingdom of Rhadaz concerning the young duke Aletto and his sister Lialla, fled from the household of their uncle Jadek, who had ruled the land of Zelharri since the suspicious death of his brother years before.

THE TWO IN HIDING
They traveled, it was said, with three outlanders who claimed to be from a place called Earth: a beautiful woman named Jennifer, who had become apprentice to Lialla in the use of the magic of the Night-Threads; the young man known as Chris, who was teaching the weakling Aletto odd but effective ways of fighting.

And, it was whispered, the third was an older woman who had the gift of shape shifting. A talent rare--and dangerous ...