Wow, this is a tough one. I'm not an expert and can't say I've been in this situation before, but I can sense something is up.
I think you need to give it a bit of time and then sit your mom and brother down for a long chat. Explore their thoughts, ask open questions. You might find out something that you would have never known otherwise. It could be something like maybe your mom feels sad/guilty that your father won't see you get married or something really deep. Or, it could be something simple like she thinks she can't afford it. I also have feeling whatever the reason your mom changed her mind, that it may have persuaded your brother to do the same.