The other bridesmaid, the one you don't like, may have initiated the exclusion, or perhaps your friend herself. If it was your friend, one possibility could be she was looking to avoid any unpleasent side issues to detract from the special event. However, for whatever reason she sided with the other bridesmaid in this instance.
Its a bit tricky. It would matter to me, how exactly the exclusion came about. There might be some detail, you don't know, that alters the picture. In other words its not possible to really judge without more info. But seeking more info, confronting your friend about it could seem petty and create the unpleasentness your friend may be trying to avoid.
I think I would probably play it cool and go along so as to not create any disturbance for your friend, and just be supportive. Take the high road. If there are any more obvious offensive actions, I would re-evaluate my participation.
Something to consider too, is that marriage often moves a person away from their friends, so this person may already be moving out of your life, and it may not be worth taking too much trouble with.
If your friend does wish to continue the friendship, I do think they eventually do owe you an apology or at least explanation of why they bypassed you.