Knowing that you will never see your son or GC again is an awful thing to have to live with Notjustaprettyface as I know all too well, because our youngest son estranged us 11 years ago.
I do understand your disappointment, especially with him failing to turn up to your D's birthday celebrations at the last minute, and that you wont be seeing them over Christmas.
Easier said than done I know, but you need to lower your expectations or even have no expectations at all. When they're invited, keep in mind that there's every possibility that they may not come. It wont stop you being upset and disappointed, but might help you 'prepare' to be let down.
It might be an idea when you are asking them round to say something along the lines of 'it will be lovely to see you all, but I'll understand if you're unable to come' (even if you don't). If your son's failure to celebrate his sister's birthday was intended to punish you, making him aware in a subtle way that you're half expecting him not to turn up, may make him wonder if his punishment of you is as effective as his thinks.
The same tactic can be applied if you're thinking about making plans for the future, including next Christmas; 'it would be lovely to spend Christmas with you all, but I'll understand if you have other plans'.
Enjoy the time that you do get to spend with your son and GC by avoiding any topics of conversation where your opinion, which of course you're entitled to have, may not be welcome. That doesn't mean you should be afraid or worried to be yourself. If your son disagrees and in so doing tells you're being rude, don't worry about politely and firmly correcting him.
I hope you'll have peace and happiness this Christmas, and that being better prepared for your son's behaviour, will help to improve your relationship and how you feel about him.