Ok. Think your getting a slightly raw deal here, because I suspect your relationship with your parents is a subject that is quite emotional for you.
You are not BU to wish that you had a lovely cosy Christmas with just your parents and DH and kids if that is your secret dream and what you feel you need. But...it's unfortunately not what your Mum needs. She obviously sees this as a wonderful opportunity to get all her chicks and grand chicks back in the nest, and who knows? She might be secretly worrying about your Dad's health and wanting to make sure that they are able to enjoy the very best family Christmases possible with him while he is still in pretty good health. And don't underestimate how incredibly excited all your family will be to spend time with you - you are obviously really loved!
I know your expectations have been dashed a bit, so don't feel bad about feeling upset, but I bet that in the event, you will have a lovely time, re-bond with all the family and create some wonderful memories that you will all be able to treasure. If I were you, I'd dig out/buy a camcorder, charge it up and film your family all together over your lovely, warm, cosy, cheery family Christmas.
And by the way, as someone who only has a very very small family left, my memories of the large family Christmases we had as children are so treasured by me - fair enough, grieve for the Christmas that you dreamed of, and maybe try and see this as a wonderful opportunity rather than a loss 