It's actually not beside the point, @princesspicker .
You and your mother are coming across really badly here.
Your mother gave her son and his fiancee money towards THEIR wedding as a gift. Perfectly normal, many parents do this for their children.
It does not make it HER event.
It does not mean that she has 'bought' the right to make demands on them regarding the 'wedding event', or to be involved in the planning.
She sounds resentful that they haven't involved her in the planning (I think we can all understand why they didn't).
Your mother probably guilted her DiL2B into letting her go to the dress fitting, on the basis that she wouldn't tell anyone anything about the dress.
She immediately told you about the bride's dress.
She 'covered' for you when your SiL2B saw your other dress and said it wasn't suitable, so deceived her.
You seem to be resentful of your SiL2B, that she is getting married when you had to cancel your wedding last year.
You feel that you 'deserve' to wear something special because it's your brother getting married, even if it's a dress you KNOW your SiL2B will be unhappy about.
You have belittled her choice of wedding dress - "OTT traditional".
You have called her controlling, demanding, and cowardly ("she hasn't got the balls to tell me herself, she's gone through my mum")
You know she won't like the dress you're determined to wear, and you don't care.
In fact, it comes across to me that you're almost gleeful at the thought that it will spoil her day just a little bit to see you in it.
Honestly, I can imagine your brother would be heartbroken if he knew you and your mother's attitude to his wedding and his fiancee.
And his fiancee would be horrified and distraught.
I can see them both going low/no contact wirh you and your mother in the not-too-distant future, and I can't say I blame them.
We all know you're going to wear the Hobbs dress, so further comments are pointless.
I hope they have a wonderful wedding, and you have the day and future you deserve.