Black Maria by Diana Wynne Jones
On the surface, Aunt Maria seems like a cuddly old lady, all chit-chat and lace doilies and unadulterated NICEness!
When Mig and her family go for a short visit, they soon learn that Aunt Maria rules the place with a rod of sweetness that's tougher than iron and deadlier than poison. Life revolves around tea parties, while the men are all grey-suited zombies who fade into the background, and the other children seem like clones.
The short visit becomes a long stay, and when all talk of going home ceases, Mig despairs! Things go from bad to worse when Mig's brother Chris tries to rebel, but is changed into a wolf .
Mig is convinced that Aunt Maria must be a witch - but who will believe her? It's up to Mig to figure out what's going on. Maybe the ghost who haunts the downstairs bedroom holds the key?