Congratulations on your coming grandchild! Isn't it exciting that the baby you gave birth to all those years ago is now giving birth to one of her own!
I've been at the births of two of my DGC and have to admit, I was thrilled to see them born! But I don't think that means everyone would feel that way. And, IMO, it's up to the parents who's going to be with them, if anyone, especially the mother-to-be - yunno, the one who's actually giving birth (within the context of whatever rules the hospital has, of course).
So I don't think it's "odd' if you wouldn't want to be there MammaTJ. And I certainly can't see insisting, even if that's what you did want. You're entitled to your feelings, IMO. And, in the end, it's all about the comfort level of the parents, again especially the expectant mother. If she doesn't invite her DM, MIL or whoever to be there, then, I believe, they need to respect her wishes.
Unfortunately, I have heard of some GPs insisting or trying to insist that they be present for the birth. So that's probably why DD and SIL "made a point" of letting you know that this wouldn't be happening. They're very they're dealing with someone like you, who wouldn't think of making such demands!
So you're doing fine, in my book, Mamma! Best wishes to you, your DD and SIL - and, most of all, that baby!