How to say "No"

Will let

Say you let them know, make other plans then say no. Why are they talking g about C in early March. They are free loading chancers. I bet they do this to others as well as it saves them money and cooking.
I would not hist them, especially as they asked so early.

Oh, not a C thread already!

"Hosting you at Christmas won't be an option this year. I'm letting you know this early so you have plenty time to make alternative arrangements." Don't give reasons why you don't want them as that risks them countering your reasons. Did your dh agree to them coming or have they assumed they are coming? To be honest, I would want him to tell them it's not on as they're his family but if he's a soft touch, then probably he wouldn't. The other option is you tell him he can crack on, but you won't be there. That's also unlikely too, if you are like me.

He has to tell them.

"From now on, we are starting a new tradition in Xmas and we will be unable to host moving forward.
We are looking forward to (going on a trip, visiting friends, whatever you want to do)"

Go away yourselves.