The Witch Apprentice by Marian Broderick
What if you were adopted by witches?
There's something very odd going on in Anna Kelly's new home - her guardians, Grizz and Wormella Mint, are witches, and they want Anna to help them with their nasty spells!
Poor Anna gets stuck in their horrible house, chopping up toadstools and scrambling frogspawn, with no-one but Charlie the cat for company. She'd much rather be out playing.
When she discovers an evil, old spell book in the cellar - can Anna make the magic work for her?