Middle grade book featuring giant spiders

I’m looking for a middle grade book I read around 2008-2009. What I remember most from the story is that the antagonists were giant spiders. I think there were two protagonists, possibly a brother and sister? I feel like there was some sort of shift into an alternate reality where were giant spiders that the protagonists had to fight, but my memory of it is so hazy it’s difficult to say. If I had to guess I would say it was around 100-200 pages long. I remember borrowing it from my local library and doing a book report about it in elementary school and it has been bugging me for years!