Given what information we have all read (we don't know what kind of relationship your M. and the bride had before this little tiff), I agree with previous posts that the bride needs to "put on her big girl panties" and "get over it". It's done. However, if she is around the same age as your brother, some of this whole issue could be a lack of maturity (no offense - she may be the epitome of maturity, I just know there's lots of... uh... not so mature girls out there, shall we say?), and if she did decide to not include your sister, she may be embarrassed and lashed out at your mother for creating a situation she cannot easily get out of.
I don't believe your mother overstepped her boundaries if she did it innocently. However, M. would be the "bigger person" and hopefully get back in Bridey's good graces by apologizing for the perceived wrong doing and be honest that she did not mean to step on anyone's toes or to hurt the bride's feelings and that (I'm assuming) she didn't know to keep quiet about the bridesmaids choices.
Weddings are a mess many times and feelings get hurt so very easily. This cat-out-of-the-bag may have taken away from the whole "bride's day" as so many say (I disagree - I think it is also the groom's day, but I love my husband), but some people's feathers just can't be unruffled. If M. acted with good intentions, remember that and try not to let the BRIDE stress YOU out over the wedding. Good luck.
Oh, and as you asked if we readers would be upset if we were the bride? I would have been like "oh... oops... okay, well that's done now, let's GET OVER IT. :)