I feel for you, Annali. I can totally see where the timing of DS's message felt like a slap in the face. It must have felt as if he were saying, "We didn't have a good time, so next year we're going away instead." That's not what he meant, I'm sure, but I understand that it might have felt that way.
As others have said, though, he might not have felt he was being "rude," but rather giving you and DH a chance to make your plans for next year. I still think he should have waited a while to tell you their plans. They could have booked their trip but told you about it later on.
I'm not sure why you're blaming DIL, however, since it was DS who sent you the message. It may have been her wish to make different plans next year. But wasn't it his choice to decide when to tell you?
"I felt wonderful and happy on leaving, thought we had all had lovely time, although I was a frustrated at how moody my DIL was throughout our stay. "
So clearly, DIL wasn't entirely happy despite all you did for them during your stay. Maybe it's b/c of her admitted jealousy of your family. Or perhaps she just doesn't like having a guest for a week. Either way, it doesn't surprise me that she and DS decided to do things differently next year. Nor is it unusual for families to go away during the Xmas season. I'm sorry it will mean you don't get to see your beloved GS next Xmas. But, hopefully, you can FaceTime w/ them and, as others have said, see them at other times during the year. Meanwhile, as time passes, I'm sure you'll adjust to the idea (as you already seem to be doing). Hope you and DH make some great plans for yourselves for next Xmas!