Help identifying a book title?

It was a book about a kid who's grandmother died and a few weeks after the grandpa married a girl nearly 30 years younger than him, I think she was the school teacher? She couldn't get married for some reason and got into a contract to take care of the grandpa in exchange for a roof over her head. The kid was the narrator and talked about how his mother hated this woman's guts and claimed she was a gold digger because she convinced the frugal grandpa to buy new appliances for the kitchen and a car. At the end of the book the grandpa died and the new wife told the narrator she was pregnant with the grandpa's baby.