The Conjurer Princess

From the back cover of the HarperPrism first edition: "Lylene's Crusade - Some girls grow up fast. Especially princesses. One day Lylene was a bridesmaid, all dressed up for her older sister's wedding to a neighboring prince. A week later Lylene as a wizard's apprentice, determined to gain the power to rescue her sister from the armed knights who had kidnapped her at the altar. The wizard gave Lylene the power to conjure a "fetch", a temporary copy of anything - a sword, a horse, even a person. An now Lylene is on the road to revenge, armed with nothing but her new magical power, and a couple of outlaw companions. Handsome outlaws to be sure. But outlaws nonetheless."