- Mythical Creatures on a farm (unicorns, basilisks, etc.).
- Relative having two children come to stay for summer.
- Children's book.
The book is called The Dragons at Ordinary Farm by Tad Williams.
Tyler and Lucinda have to spend summer vacation with their ancient uncle Gideon, a farmer. They think they're in for six weeks of cows, sheep, horses, and pigs. But when they arrive in deserted Standard Valley, California, they discover that Ordinary Farm is, well, no ordinary farm. The bellowing in the barn comes not from a cow but from a dragon. The thundering herd in the valley? Unicorns. Uncle Gideon's sprawling farmhouse never looks the same twice. Plus, there's a flying monkey, a demon squirrel, and a barnload of unlikely farmhands with strange accents and even stranger powers. At first, the whole place seems like a crazy adventure. But when darker secrets begin to surface and Uncle Gideon and his fabulous creatures are threatened, Lucinda and Tyler have to pull together to take action. Will two ordinary kids be able to save the dragons, the farm—and themselves?
Might this be Fablehaven by Brandon Mull as per Book: Girl can see magical creatures, finds secret passages at her grandparents'?
The series begins as 13-year-old Kendra and 11-year-old Seth Sorenson are traveling to their Grandpa and Grandma Sorenson's house while their parents are away on a 17-day cruise. When they get there, they also meet Dale, the groundskeeper, and Lena, the housekeeper. Grandma Sorenson is "mysteriously" missing. Grandpa Sorenson does not tell Kendra and Seth about Fablehaven being a secret preserve for magical creatures at first, but instead sets up a rather complex puzzle involving six keys and a locked journal for Kendra to solve. Once Kendra unlocks the mostly blank journal, she discovers the words "Drink the milk." She and Seth drink the magical milk Dale sets outside in their yard every morning, opening their eyes to a whole new, mystical world full of the magical beings of Fablehaven. Then Kendra and Seth must face challenges such as defeating an evil witch and a powerful demon, defending the preserve from an evil society, stopping a plague that changes creatures of light into creatures of darkness, accidentally turning Lena into a naiad, and, ultimately, protecting the world from a horde of imprisoned demons.
The first book came out in 2006, which matches your timeframe.