The Fifth Sorceress

The Fifth Sorceress is a great novel by Robert Newcomb. It is a book about a great nation recovering from an attack made hundreds of years previously. They were attacked by Sorceresses using the Vagaries (evil side of magic). Being able to use only the Vigors (good side of magic) themselves, the wizards were nearly defeated.
Hundreds of Years later the sorceresses are gone. Prince Tristen and his twin sister Shailiha (I may have spelled that wrong) are called the chosen ones. They "find themselves thrown into a world they hardly know and yet must save."