Ma soeur est une sorcière

An elegant stranger appears on seedy Coven Street and collects the two Chant orphans -- Gwendolen, a powerful young witch, and Cat, her brother. The stranger, known as "Chrestomanci," delivers the children to the grandeur of Chrestomanci Castle. Frustrated that the witches of the castle refuse to acknowledge her talents, Gwendolen conjures up a scheme that could throw whole worlds out of whack.

These can be read in the order published, or the author has suggested a different reading order, found here: https://www.howtoread.me/chrestomanci-order/