This is a tricky one. I would extend the invitation and maybe make it clear that this year is special (because of xyz) and that it would be particularly lovely if they could join you this year, but that you understand DS's GF family also might have calls on her time, or that next year they may want to spend with her family. i.e. be warm, enthusiastic, but don't apply pressure.
I'm very close to my family, and my DBF is close to his. Which is great...up until the point that our mothers (and it's largely them as opposed to fathers) get upset that we're not at every family event. Christmas last year was a nightmare - we spent every night for a week at either his or my parents' house having the same conversations with different family members (same city, but still..hardly a holiday). When his mother applies too much pressure on us to attend something it just sets my hackles up (I can tell my own mother 'no' but it's harder with his), and makes me not want to go, whereas if I know it's a special event (and particularly if it's one I know that DBF would like to go to) I'll make the concession. Also, if he decides to not come, try not to blame his GF (not saying you would, just from my own experience). BF's mother does this to me (when in fact it's BF being either lukewarm/not telling me about it and I've made plans/being indecisive about whether he wants to go), and it really puts me off spending more time there. (obviously that's just me - your son's GF may be nothing like that).