LOL - I must admit I'm also quite amazed that everyone is of the same opinion!
I should have mentioned in my OP that not visiting is not an option, because DH wants to see his family for the holidays too. And despite the holiday hiccups in the past, I do genuinely like BiL and his GF. (They're not married, but they've been together longer than DH and I, so treated as one would a married couple).
I suspect that part of his controlling "must be done my way" behaviour is due to his upbringing: he was the Golden Child (firstborn AND first son), their sister is Baby of the Family and Only Girl, while my DH is the Poor Unloved Middle Child. Stereotypes? Yes. True? Also yes. DH and BiL have had their difficulties in the past (and I'm often a sounding board for DH to vent about him!), but these days they spend a lot of time together in community and sporting groups they both enjoy.
I spoke to my mum about it this morning, and her advice was to just spend the day with BiL and DH's family - my family will get together either on Christmas Eve (my preference) or Boxing Day. We'll see my grandmothers on Christmas Eve at my aunt's house, so it'll all work out. It may be a little spineless taking the path of least resistance, but for long-term family harmony it's for the best (because it won't just be BiL who'll moan if we don't turn up!) This isn't my hill to die on.. yet.
Family politics are horrible, especially when you get along with them all!