I actually think it's rather off of your sister to bring this up now.
If the assumption was that lunch would be served at lunchtime (because that's how your family has always done it, for example), and arrangements to help her out with the cooking were made based on that assumption, it seems rude to me to suddenly change that just a few days before Christmas Day.
I must admit, I wouldn't be happy about a 6pm mealtime, if only because my children have always been up by 6 in the morning and frankly, by 6pm I'm ready for bed! (well, not quite, but near enough

In your position I would just explain about the kids being fractious in the early evening, you'll have your own Christmas lunch at your place and pop round later. Depending on what part of the meal she is expecting you to cook, could you bung an extra tray of roast potatoes/veg/whatever in your oven after cooking your own meal for you to take round to hers later for reheating?
Or offer to provide all puddings- you could have pudding together at hers? That wouldn't take too long. They could have their main course before you arrive, maybe- providing the BIL's flight is on time, that he wants a full-on roast etc etc