So sorry for your loss.
I fear that if you go and just "go through the motions" you will just seethe with resentment, have a terrible time and in your heart, never forgive them.
To be honest, I would absolutely forget about them right now. They did nothing - and so you owe them nothing in return!!!!
Their invite at Christmas is just forcing you to see them when you are grieving and not well and have reason to be mad at them.
Why do you owe them a Christmas together? You do not.
Put your relationship with them in a drawer for now. Come back to it later when you feel a bit more yourself.
Your husband can explain that as it is the first Christmas without your dad, you just want to be at home. There's nothing bitter or confrontational in that. If they want to express their condolences at that point, let them, thank them, and then still spend Christmas at home.
Regardless of their shitty behaviour it would still be 100% understandable if you didn't want to go, even if they had been the epitome of sympathy.
This is going to be a hard Christmas - please, OP, put yourself first, take a stand and just have it at home.
Don't make yourself do this.