Probably written in the 60s or 70s, this is a YA book about a pair of spoiled siblings growing up in the city (I think the opening scene is of them terrorizing a babysitter). Their parents announce quite suddenly that they (the parents) are going away for the summer and that the kids will live with a relative (an aunt or a cousin) in the country. Their aunt has a husband or boyfriend who I think is Native American and the kids are both afraid of him and passionately unhappy, especially since they are expected to help fix up the big old house in which they are all living. Eventually they start working (I remember clearly a scene in which they scrape all the old paint off of tall pillars out front) and gradually are reformed so that their parents hardly recognize them when they return.
Stump the Bookseller
New format, same book stumper solving.
This is the one: “The Atrocious Two” Sheila Greenwald (1978). A brother and sister who are “perfect brats” are forced to spend a summer at Aunt Tessie’s where they find out some surprising things about themselves and become involved in a jewelry theft.
The twins are Ashley and Cecilia and Aunt Tessie’s Native American husband is called Bear. They have to help fix up the house so she can turn it into an inn.