Tomorrow's Fire by Linda Yarroll

Twins Kylie and Karver were born not knowing they were wizards. They discovered this when war threatened the five realms. Their father wizard Drule, who had been living with them as a simple Smith, now had to teach them how to use their gifts. Kylie learns that she is the mistress of dragons, while Karver strives to learn exactly what his role will be in the upcoming war.