Fantasy Siblings Stop Time

Hi everyone,
I remember reading a fantasy book about siblings - I think they were staying at their aunt and uncle's house in the country? They met a mysterious groundskeeper who gave them (I believe) a necklace, and when they woke up in the middle of the night, they discovered time seemed to have stopped for everyone else. They were pursued by some kind of knights or soldiers on horseback through the estate grounds, I believe it was snowing or winter at least. I have some memory of them crossing over a bridge... I don't know how the story ends, I think I had to return the book before I finished.
Thanks!

Partial overlap with The Ghosts by Antonia Barber, though that's a long shot.

>2 beyondthefourthwall: Definitely has some overlap, but that's not quite it.