Goats Don't Brush Their Teeth (Abby and Tess Pet-Sitters)

"This award-winning series features Abby, an aspiring veterinarian who desperately wants a pet but lives in a "no pets allowed" apartment building, and her younger sister, Tess. The girls go into the pet-sitting business and discover new things about animals and new ways to solve problems.
In Goats Don't Brush Their Teeth, Abby can't wait to spend a week at her grandmother's farm. There's a new animal to take care of, not to mention her own bedroom free from her annoying younger sister, Tess. But despite her new freedom, Abby begins to wonder where Tess disappears to very day, and why things around the farm keep going missing. As the mysteries slowly reveal themselves, both sisters learn that keeping secrets can sometimes be more trouble than it's worth."