I had somehow never read Diana Wynne Jones - I know, I know! - so when Sarah Rees Brennan recommended The Ogre Downstairs I thought I'd give it a try. It's about two families of siblings who are suddenly step-siblings and trying to figure out how to make the new family work, and the new stepfather is horrible (they think), and there are magical chemistry sets wreaking all sorts of havoc. Honestly, it took me a while to get into it. Well, it was a perfectly pleasant reading experience from the start, but I would read a few pages before bed, put it down, and not feel any particular call to pick it up again until the next night. But I think that's more my problem than its, and when I finally got a decent stretch of time when I wasn't falling asleep, I read the last half of it in one go. The magic is very well grounded in the real world, and the way it comes to help the kids recognize that their new family members aren't necessarily as awful as they seem is really well done. My favorites were Malcolm, who comes across as mean but maybe is just shy and lonely and fragile, and the little one, Gwinny, who overcomes her fears with bravery and honesty and open-mindedness, and is much more of a real character than little siblings in YA books often are.
Sadly, this seems to be out of print, but I'm sure your library can get it for you!