" I truly think it’s her attention they are seeking..."
I agree. What isn't clear, however, is if this happens when they are just w/ DD alone. Maybe in company she thinks she can relax and focus on herself a bit? Perhaps she's counting on you and her siblings to give the children attention when she's w/ all of you? Not saying she's right, just that this may be her viewpoint. As notanan suggests, since the girls know how to behave, she must be teaching them when they're alone.
She's your DD, if you're not sure how to talk to her about this, I take it you know she's sensitive and might resent it or just brush it off as she does w/ her friends. She would probably resent your trying to discipline her kids, as well. But she might be ok w/ and actually appreciate your bringing things for the kids to entertain themselves with, as a couple of posters have suggested (coloring books, etc.). Really, IMO, DD should do this, but since she doesn't, maybe you could.
I also agree that some children have difficulty handling it when adults are only engaging w/ each other and not them. So it may be that they're not just looking for DD's attention, but that of the adults, in general (DD, your other AC, yourself, etc.). Perhaps you all need to focus on the GC and leave the adult conversation for a time when the kids aren't around?
If you don't feel any of this will work out, then it may be best to stop having these family get togethers until your GDs are older. Or maybe if your other AC have kids at some point, they'll become more sympathetic to their sister. But for now, it seems to me, these gatherings just aren't working.