@Wantsummerback
Oh, this is the SiL from that 'SiL thread'?
Well, you must have sort of suspected this would happen based on past experiences.
I can understand your frustration, but once you give someone a gift, unless you've set conditions on it, it's theirs to do with as they wish whether you resent it or not. For that, I think you need to just grit your teeth and remember never to gift your iLs any kind of trip unless it's locked in to the number of 'guests'. That way, if they want to let SiL horn in, they'd have to pony up the difference.
The cabin isn't costing you more, so I'd be able to let that go unless there's a 'linens charge' or something.
As far as the chef, I don't know exactly how that works. Wouldn't you pay ahead for the number of people being catered for and he/she buys the food based on that? Or do you hire the chef and he/she then bills you for the food? But in either case, yes, I'd definitely tell the chef that you are paying for food for TWO only and that if any 'extras' show up, you are not paying for them and he'd have to collect from the iLs. And then I'd tell your iLs exactly that.
Or you could go for the 'nuclear option' and cancel the whole trip citing SiL as the reason. Then rebook something she can't horn in on.