Guardian of the Gauntlet by Lenita Sheridan

Camari and Mila are two sisters who are princesses and live in the kingdom of Harroway. One day they encounter Prince Denir from the kingdom of Thalon. He was given a special gauntlet that has special powers, but only when used by one who has a belief in a higher power. One day he turns Camari invisible. A problem occurs when Denir is called off to war and Camari is left invisible. Also there are two villains who are conniving to steal the gauntlet. What ensues is a marvelous adventure.