The Broken Dragon

"Full of life and energy, Lily has great ability with plants and their healing properties, but no magical talent. In contrast, her beloved twin sister Valeria has unmatched magical abilities, but is frail and often ill. Since birth the two sisters have completed each other, but the older they become, the more apparent it is that magic is taking its toll on Val. Together the sisters embark on a journey to solve Val's magical problem, and along the way to meet their mentors, the girls encounter several other magic users also struggling with severe weaknesses"--Amazon.com.