"...at least they're not suggesting that you cancel."
I was thinking this ^ exactly. And that, TG, they didn't wait till you were there to tell you! I would be disappointed, also, I like Halloween but I'm not into camping. But I would focus on the fact that I'm getting to spend time w/ DS and my GC and try to make the best of it, as some have suggested. I hope scribbles idea can work out for you. If not, I suggest just putting up w/ it all for the sake of enjoying DS and family.
Perhaps it will help to remind yourself you'll be at the comfort of your brother's after the weekend. And next time, please ask if they've checked their calendar before you actually book your trip, LOL. I know you shouldn't have to, but, IMO, it would be a good way to prevent this from happening again. Or try not to go there in October in the future, LOL!