First, remember that you cannot reason with a crazy person, they'll drag you down to their level and beat you with experience.

The first thing to do would be to not tell her when its time. The second thing to do would be to not tell any relative that might break this trust and tell her behind everyone's backs.

Your brother needs to grow a spine and defend his wife and mother of his children. He needs to leave orders that the aunt is not allowed in the delivery room and should be ready to call security if she tries to get in.

This aunt sounds like one of those pathetic people who always has to be the center of attention and behaves like a spoiled brat when she isn't. Don't bother arguing with her.

Best of luck on the blessed event - I hope it can be that this time.

Finally, consider showing this thread to your brother. To the brother - congratulations on becoming a daddy again. Now defend your wife and child and don't allow the maniac aunt to ruin your day. Think of how appreciative your wife will be for the rest of your life if you can rise to the challenge and be her knight in shining armour.