Both of my brothers eloped without notice. This was back in the days when if a gentleman got a lady with child, he married her, so exact wedding dates may or may not have been fudged.
Times have changed, so I doubt this is the situation with your brother. Come to think of it, my sister eloped for her first marriage, too.
While reading this thread, I've been trying to remember how these announcements went over. I will say that a big reason for keeping our mother out of the loop is that she would have disapproved of my siblings' marriages, if not of the new in-laws (sister was too young, one brother had just started college, other brother was perceived as irresponsible). No hysterics, but there would have been some stern lectures. If your mother is into appearances, my guess is that your brother fears that the woman he loves might not measure up.
While my sister's marriage didn't last, both brothers have been happily married for many years, and I have to say that their wives were good for them. So, please keep an open mind when you meet your new sister-in-law. At least your brother made the announcement via Facebook rather than a snarky postcard, or an introduction at a funeral.