You often see MIL who expect to be present at the birth of GC if the DIL's own mother is asked, on grounds of "fairness". It always amazes me, because the assumption that the relationships between parents and children are not biological plus years of love and nurture, and that a marriage or long term relationship means a woman has an equal obligation to her partner's mum, is bonkers. Sure it's the birth of a baby, but it's also a labour, and the birth of a mother. And there is in such cases absolutely no recognition that the woman and her mum have a unique bond, and it isn't the MIL's son giving birth. And that assumption that a relationship with the son devolves rights to his mother, over the life of the DIL, seems at the heart of a lot of problems.
I tend to think it's because women are always supposed to service family connections, and the idea that a son should sort out the relationship between his parents and himself and his kids, rather than the DIL being saddled with it, a remnant of the old idea that women married into another family.
But this is the same thing. Your DD's partner is his parents' son and his siblings' brother. They (hopefully) love one another and share a history nobody else ever could. Why should your DD be part of that? It's lovely when someone marries into a family and develops a bond with the extended relatives, but it isn't automatic and instant, and the reality is that the rest barely know her - she's his partner, not their child or sib. Of course they like having a way they can chat and communicate without having to check their words or parse their statements. And that's not possible when someone new is involved. They aren't rejecting her, just because they want to speak sometimes with their own nuclear family. Over years, she may join them in that intimacy - or she may not. What matters is that she develops it with her own partner and children for herself, as is her right.
I think your DD needs to stop and have a think. She'll soon be a mum herself - would she really like it if, 30 years ahead, her child's partner won't accept that she may want a loving, close and private relationship with her kids, and their father, to continue... without that upsetting a newish partner?