I think, or should I say I 'hope' that the reason they're staying in alternate accommodation is because they realise that you will have a newborn and all that goes with that.
So to that end, hopefully when they do come around to your place, they will help (I know you stated earlier they're not known for that) but maybe when they see the baby and all the hustle and bustle of the festive season they may change their tune. Even if they entertain your other kids, it may help ease the load.
(((Hugs))) none of us can tell you how you will feel/cope, or what you can handle only you know that, but I'm sure most of us can empathise with situations that may cause great anxiety beforehand.
I do think it's a little odd they didn't at least ask what YOUR Christmas plans if any, were.