SOLVED. YA/Children's Fantasy series, 4 books. MC's are a boy called Seth and his sister who go to their uncle's house. Mythical Creatures such as Nymphs go wild on midsummer's night. [s]

[Florence]

Seth and his sister (I can't remember her name) go to a relative's house and there are lots of mythical creatures such as nymphs and centaurs in the forest that go wild on midsummer's eve/day.
The author is an ex-New York times editor.
The series title has the word 'Haven' in it, I think. It's a YA fiction fantasy that I read in 2016.

[Jannell]

Fablehaven by Brandon mull

[Florence]

Thank you, this is it

[Kris]

Glad you found your book, Florence. Here's the link for Jannell's find - Fablehaven by Brandon Mull.