I have a family member who is notorious for being chronically late to everything. Except things I'm in control of.
I don't fuss, I don't fret, I just let them know what time x will be, and when that time comes, I proceed accordingly. If they're not ready, that's okay, I will proceed without them. They know that I mean what I say, and I say what I mean.
My mother and sister are mystified as to how I never spend my life waiting upon this family member. It's quite simple. They know I won't. I won't get in a lather, I don't get angry, I don't let them control my actions.
So I'd be saying 'just so you know, whether you are here or not, we will be eating at x time. We'll be very happy if you're here to join us, but if you're not, that's fine. But be very clear, we will be sitting down to eat at this time, with or without you, and if you arrive mid-way through, you will need to sort yourselves out, because we will be enjoying our Christmas meal. If you could let us know before hand whether you will be joining us, that would be ideal, but if you don't, and you're not there, we'll be assuming that you are not, and proceeding accordingly.'