Could you sneak up behind DH when he's on the phone to them and say 'oh, have you asked MIL and FIL about Christmas, would they like to come?'
I know it's a bit underhand, but they will be bound to have heard you in the background, so DH will have to ask! He can say, 'I almost forgot, thanks for reminding me darling, I knew there was something I meant to ask' You could plan the phonecall and use this as a script, holding his hand while he asks them. At least you will know that they have been asked. Ask DH, what would be worse, not making your position clear by inviting them, or letting them hope that not talking about it means that you might have relented and will go to them after all? DH said last year that it would be the last time, and so this year he needs to remind them.
I think I remember the post from last year, isn't your MIL the one who likes to be queen of Christmas, and has to do it her way, at her house, and everyone has to come? And SIL is a bit immature and self-centred?
You know what, you can ask them and give a final date by which they can accept your invitation or decline, and if they don't let you know either way (which would be very rude) you assume they won't be joining you and get on with your own plans and preparations. If you go to PILS on Boxing day, let her do whatever she wants to do and just eat what you feel like. Regard it as one less meal for you to cook, and leave when you choose.
Whatever you do don't give in!