the Time of Green Magic by Hilary McKay
When Abi's father marries Max and Louis's mom, their families start over together. Abi suddenly finds herself the middle child, expected to share far too much'especially with grubby little Louis. Then they move into an eerie, ivy-covered house, big enough for all of them. But for the children, strange things start to happen in that house. Abi reads alone, and finds herself tumbling so deep into books, they almost seem real. Louis summons comfort from outdoors, and a startling guest arrives'is it a cat or something else? Max loses his best friend'and falls in love. Meanwhile, Louis's secret visitor is becoming much too real. Now Abi, Max, and Louis must uncover the secrets of their new home'for there can be danger in even the most beautiful magic.