By Hilary McKay.
This is another one of those books I should have found years ago. Serious, sweet and hilarious. The notorious driving lessons are easy to point to as one of the amusing points, but fried corned beef sandwiches and curry sandwiches ("Should I make the curry very hot or very, very hot?") amuse too, as do most of the things this quixotic family gets up to.
The Casson family is a family of artists, with colorful children: Cadmium, Indigo, Saffron, and Permanent Rose, and parents Bill and Eve. The trouble starts when Saffron, perusing the color chart tacked to the kitchen wall, finds all her siblings' names but not her own. The explanation? She was adopted. Oh, tragedy: suddenly she feels like she isn't part of the family at all. One of the strange things about this book is that after this happens, about five years pass in the space of a couple pages, five years where Saffron feels alienated and her family continues to put up with her, not to mention love her.
The family is eccentric but still a family: they come together when they need to. When Rose finishes her first drawing and the "wicked teacher" who had pretended interest snatches it away and stakes it to the wall far above her reach, Cadmium helps her to steal it back and replace it with a replica, down to the four thumbtack holes. And when the wheelchair girl (quite intentionally) runs Saffron over, she suddenly has the friend she didn't know she needed.
Sweet story, nothing too heavy, lots of humor. Not what most people would call fantasy.
Found via E. Wein, who offhandedly mentioned the series, with a quote, and Sherwood Smith.