Peppermints in the Parlor by Barbara Brooks Wallace, 1980.

Newly orphaned Emily Luccock journeys to the home of her Aunt and Uncle Twice in San Francisco. She has not seen her aunt and uncle in years, but they are the only family she has now, and she hopes that she will soon feel at home in their lovely mansion, Sugar Hill Hall.

Upon arriving in San Francisco, however, she is met by her Aunt Twice, who seems to be a pale shadow of the woman she once was. Sugar Hill Hall is now owned by the sinister Mrs. Meeching, and Aunt Twice is forced to work as a cook and housekeeper. Aunt Twice seems very frightened of something, but she refuses to say exactly what she fears, and Uncle Twice is nowhere to be found.

Aunt Twice tells Emily that her wealthy parents lost all their money before they died, and she is now poor, too. Emily is quickly put to work as a housemaid alongside Tilly, another orphan girl. Her only real friend is Kipper, a fishmonger’s son who is paid to do chores around the mansion sometimes. The house is being run as a home for elderly people, but the elderly people are treated badly, fed rotten food, and punished severely for even so much as taking a single peppermint from the bowl in the parlor. Emily tries to do little things to make the old people happier, but all the time, she is right at the center of all the sinister events surrounding Sugar Hill Hall.

The book is available online through Internet Archive.