Power of Three
by Diana Wynne Jones
DWJ is so, so, so good at emotional development and family dynamics. The only sour note was a minor but persistent thread of fatphobia. Brenda even gets development and a heroic arc of her own, but for some reason (I guess: being published almost 50 years ago) we can't have that without the narrative constantly making vicious little jabs about her every time I thought it had finally been dropped.