SOLVED. Children's chapter book: Two middle-aged or elderly sisters are the villains (one short and plump, one tall and thin) [s]

[Maggie]

Hi all, our teacher read this children's chapter book to my class in the fourth or fifth grade of elementary school (1994-1996). I wish I could remember more details, but unfortunately mostly what I remember is the feeling that I had when she had finished the story (which was, "Wow, what a great twist!").

The hero of the book was a young child (I cannot remember whether it was a girl or a boy). The child somehow ended up living with or working for a pair of middle-aged or elderly sisters. One sister was tall and thin and depicted as cruel and strict--the reader had the impression that she was the villain. The other sister was short and plump and depicted as kind and warm--like a nice grandmother. The reveal at the end was that the evil mastermind was actually the short/plump/sweet sister, who readers discovered was only acting kind and grandmotherly in order to deceive the child hero and others. The tall/thin sister turned out to be a push-over, just following her meaner sisters orders.

The cover of the book was hand-drawn figures of the two sisters (and possibly the child hero) in Quentin Blake style (they were cartoon-like, not realistic).

Any ideas? Much appreciated!

[Cupcakedoll]

Could it be this one? I feel like I've read the book you're describing but so long ago I'm not sure if this is it or not.

[Maggie]

Thank you!! That's the one!