Book Circa 1970's-2000, probably YA, fantasy, medieval style, FMC.

The book was read by me around 2002ish but our school library didn't have the newest books so....

The book was from the viewpoint of a FMC who was I think the daughter of either the head person in her village or possibly she might have been a Princess. Her and her younger brother are to go to another place to visit and before they leave her grandmother(?) gives her a special Scrying Stone to keep. While they're riding through the woods they're attacked and the Scrying Stone shows the FMC and her brother a path away from the enemies. I wanna say the stone may have been light in color, perhaps even cats eye/moonstone esq but that might not be the case.