They don't owe you any particular date to deliver this message. There are multitudes of reasons they may have delayed in announcing her pregnancy (assuming she is, to begin with), some of which are probably not your concern.
How would I react? To me, any announcement of a new grandchild in the family is marvelous, happy news. I would express my delight.
If they hold back on communicating with you, could it be because you are conveying a hostile attitude? It comes through loud and clear in your post. You're making it very clear that you don't like her.