What's the Name of That Book??? discussion

It's part of a series, and there are atleast three books. I can only remember a little bit. The man characters are the brother and sister, and every year during the summer their parents drop them off at their grandparents house, which is actually a mystical fantasy creature haven. In the first book, the siblings are given the room in the attic and they soon discover a key and locks, and they have to find all the keys and solve the puzzles etc. to unlock the mystery item. In another book someone named Vanessa/Veronica is told to drive the kids to the plot, and in the end of that book (view spoiler)[ she betrays them and they end up locking her in some dungeon. I believe they had captured a bad guy down there and she was trying to free him but I could be way off. (hide spoiler)]
I believe there's an all knowing Spyhnix as well but I don't remember how he ties in. There are centaurs and pixies on the manor as well.
Read this at the end of 2018. Not sure when it was written.

Fablehaven

That's it, thanks!