Blood runs thicker than water. Your brother will always be your brother, but will your wife always be your wife?
Granted that is a somewhat cynical view, but ask yourself this - if you should break up with your wife in 6 months time, would you regret not going to your brothers wedding?
He is your brother, and not only that he wants you to be his best man on what should be one of the most important days of his life. Newborn or not, if your wife was worth having she would completely understand why you need to be there.
True, his marriage may not last, but he is still your brother.
As others have said, perhaps go out for a short period and come back? That would most likely be my decision.
As for your mother-in-law - I would have a quiet word and tell her to keep out of your family business.