You can't fix it. They made their own bed, they can lay in it.
Tell the folks who are interested - your DH's grandmothers, sibs, whoever. Give no further thought to those who aren't interested - they're not giving you any! If they don't want to hear about it through the grapevine, or if they're surprised to learn that they've got a grandkid at the next family gathering, well, tough - you're just respecting their wishes by not contacting them directly. They don't get to eat that cake and have it, too. And I POD the PPs who have suggested that keeping those two in your life at all is not likely to benefit your child in any way, and could be harmful - they sound like walking hazardous waste dumps, and your first priority now needs to be the health and well-being of your child. If they come back from their vacay and pitch a fit about not being the first to know, or finding out through the grapevine, then by all means feel free to give them a cut direct yourself.
Congrats, and best of luck with labor & delivery, and those first few sleep-deprived months. :-)