I think I would go with B. "Congratulations! Welcome to the family!" but not actually faking surprise--hopefully there will be things to get excited about (how did you meet, cute kid, beautiful ring, etc.) that will make the woman and her son feel welcome and make the meal go smoothly. Actually, much more smoothly than if it had really and truly been a surprise, and you were left stammering and blanking on what to say next, getting angry, etc.. Plus, there will be other guests at this meal for whom it is truly a surprise, right? So if Bro is looking for a jaw-hits-the-table moment, he can get them from other people, while you have prepared in advance to smooth over rough spots.
You don't have to tell Bro it was Mom who told you (I would bet he could figure it out), and if he chooses to get mad that you didn't faint dead away with shock and make the entire dinner extremely awkward, then so be it. I just think his behavior is rather silly; maybe you could try to elicit what he thought would happen if it was a total surprise to everyone, and then paint a more realistic picture for him of what would have happened, if you hadn't been in the know.