Author: Diana Wynne Jones
Eric and Gwendolyn Chant are orphaned when their parents drown in a boating accident. Gwendolyn seems to have inherited all of her parents' magical powers while Eric (referred to as Cat by his sister) appears to have none. Eventually, they come under the care and tutelage of Christopher Chant, better known in the wizarding world as Chrestomanci.
Being a Miyazaki fan, I am more familiar with Jones's Howl's Moving Castle series. Charmed Life was actually written first, though anyone who knows the Howl books will notice common narrative elements. For starters, Howl and Chrestomanci are similar characters: tall men in flashy clothing, powerful wizards, not especially verbal, intimidating to everyone around them. Travels between parallel worlds are important to both stories and in both cases, one of the worlds is our own.
Now that I've read it, I'm a little surprised at my daughter's fondness for it. Not that it's not good. It is good. But for a long time, I've worked under the assumption that she preferred stories with female protagonists - not the case here. In fact, Gwendowlyn turns out to be highly objectionable indeed. My daughter does love stories about magic, though, and there's plenty of it here. It's a fun read. I'm definitely interested in reading more of the series.