You're very welcome, Madgran. And thanks for taking time to answer us individually.
You say this is about DIL not wanting to spend time w/ her ILs /you and DH, etc. But it does seem that she often keeps DS away from you, too. It could be coincidence that there so often seems to be a last minute emergency or whatever. But if it has happened 3 times or more, I admit, I would be suspicious that she's making sure it happens, especially if those times were close together. So really, it is, after all, about her trying to stop DS from seeing his FOO (family of origin). Or maybe he doesn't want to come to certain events and doesn't have the heart to tell you.
" I meant the potential for "accidental" estrangement or at the very least, never or rarely seeing ones parents...see above. "
Let me see if I understand this ^^. I think you're concerned that if you stop inviting DS, you and he will end up being estranged. And, by extension, you'll end up being estranged from the GC. Have I got that right?
I know you said there's "not much point" in even discussing this, but Iv been thinking... If Iv got it right , then, hmmm... If you keep in touch in other ways - Skype, etc. - then his not coming to events or for visits won't really cause an estrangement. If you don't keep in touch that way, would it be worth starting?
If Skyping, etc. won't work out, then continuing to invite him to this/that would be a way of keeping the lines of communication open. You'd invite him, knowing he will probably renege, but, at least, you'd be talking w/ each other.
Whether videochats, etc. will work out or not, another option, I think, would be to let him know that you realize it's often hard for him to go through w/ plans w/ you, so you're going to stop asking. But that he should know the door is always open and that he should let you know if he ever truly free to see you and DH, etc. That could be hard to say w/o sounding confrontational, I know. So I would think it over carefully before I said it.
Maybe someone else can come up w/ a better wording or a better idea.