Diana Wynne Jones said this quote

“ I think it was then that it dawned on me that Mum wasn't going to notice Chris was missing. She has been made so that she thinks Chris is just round the corner all the time. She doesn't realise that she never sees him. I don't know why I didn't understand earlier. If Aunt Maria can turn Chris into a wolf, she's surely strong enough to do this to Mum- except that it seems a different kind of thing, much more natural and ordinary, and I didn't really think she could do both kinds. ”