It's a Baby, Andy Russell (Andy Series #6) in Bloomington, MN

While Andy Russell's parents are at the hospital with his newborn brother, Andy is having big problems at home. His strange aunt Janet has come to babysit, and she's on a mission to rid the house of germs in preparation for the baby. Andy is convinced his aunt wants him to clean all of his "germy" pets . . . right out of the house!
The sixth book in the Andy Russell series is filled with hilarious misunderstandings, warm reunions, and a sweet new addition to the Russell family.