Diana Wynne Jones Wiki

In The Merlin Conspiracy, Isadora Dimber is one of the Dimber twins, possible heirs to the youngest position in the Maiden/Mother/Crone group that constitutes the core of this female clan of witches.

Isadora and her twin Ilsabil are identical, highly theatrical, very manipulative, and irritating to everyone they come in contact with, except their mother and grandmother, Judith Dimber and Hepzibah Dimber, who are under a glamor spell to make them overlook any evidence of the twins' misbehavior.

Their cousin Roddie (Arianrhod Hyde) and her friends end up calling the two girls 'the Izzys' because it's so hard to tell the two of them apart. 

When the hereditary witches of Blest (including Isadora's mother, grandmother, and grandfather) are all spirited away into captivity as part of the conspiracy, the Izzys hitch a ride with a neighbor to the grandfather's house, where they become part of the group that is traveling from place to place, trying to oppose the conspiracy and rescue all the lost wizards and witches.  Although infuriating at all times, on a couple of occasions the Izzys manage to save the day -- something they are very very smug about.