Ogre Downstairs

Diana Wynne Jones at her finest — family feuds and chaos, magic with hilarious results and some of the most original ideas ever to appear between the covers of a book.
Casper, Johnny and Gwinny get a big shock when their mother marries the Ogre. The Ogre is large and stern and not at all interested in children, although this doesn't prevent him from adding his own two awful sons, Douglas and Malcolm, to the family mix. Now the five children and two adults are squashed under the same roof, which can lead to only one thing — war!
Then the Ogre brings home the Chemistry Sets — one for Malcolm and one for Johnny. Not that Johnny is impressed by this very obvious bribe. At least, not until they accidentally discover the flying lotion. Then the real fun begins…