Fudge-A-Mania by Judy Blume, 1990.
Peter is horrified when he finds out that his family is going to spend their vacation in the same place as bossy know-it-all Sheila Tubman and her family. Even worse, the two families are going to be staying to be staying right next to each other. Really right next to each other. They’re staying in the same house, which has been split into two halves. As far as Peter is concerned, the only thing that might save his summer is that his friend Jimmy will be coming up to stay with them part of the time.
The arrangement turns out to be a little better than Peter thought it would be at first. Sheila finds a way to make some extra money by baby-sitting Peter’s five-year-old brother Fudge. Fudge says at first that he wants to marry Sheila, although it turns out to be mostly because he’s afraid of monsters in his room at night, and he thinks that if he gets married and shares a room with Sheila, it will keep the monsters away. Then, he decides that marriage may be unnecessary when he makes friends with a little girl named Mitzi, who is staying with her grandparents nearby. Mitzi’s grandmother makes a special monster spray for her to keep monsters away, so Fudge decides that he might not have to marry Sheila after all.
Peter is happy when he discovers that Mitzi’s grandfather is Big Apfel, his baseball hero, and that he holds baseball games that are open to the public, so he and Jimmy can also play with him. He also gets a crush on Isobel (“Izzy”), a girl who works at the local library, although Isobel is a few years older than he is. The baseball game goes well enough, but the crush, not so much.
Then, comes the most shocking news of the summer: Peter’s grandmother and Sheila’s grandfather decide to get married! If that happens, Peter and Sheila realize that they’ll be related by marriage!
Big Apfel and his granddaughter Mitzi are fictional characters, but the book that Mitzi claims is about her, Tell Me a Mitzi, is a real book.
This book is part of the Fudge Series.