Nora and the Texas Terror
Reading Level: Grades 2-4
Nora's life is turned upside down when her uncles family move in with her family and Nora has to share her bedroom and school with a cousin she has never liked.
For another tale of an obnoxious relative try:
The Diary of Melanie Martin, or, How I survived Matt the Brat, Michelangelo, and the Leaning Tower of Pizza by Carol Weston