most of the comments have been about my wedding and how much it's cost which isn't th point
I agree OP - it isn't the point at all.
T my mind the post is that your wedding has been carefully planned and saved for (I wouldn't spend that much myself, and obviously nor would many on here, but IT ISN'T OUR WEDDING SO NONE OF OUR BUSINESS! You have a soon-to-be-SIL who has a history of playing unpleasant and cruel "jokes" and now she has decided to get married very shortly before you in a distant destination (her wedding, her choice). But I do think she is being deliberately bolshy and trying to steal your thunder.
It is none of your mother's business who pays for your wedding, nor is it her business whether or not you go to your brother's. If she's that bothered, let HER pay for it - it's not for her to tell other people how to spend their money, and I think that you and your DP taking full responsibility for all the costs is a very adult thing to do. You aren't children, and you have been together for some time - your responsibility. I'm sure your dad will offer what he can as a wedding gift, and he is in a situation where he can only offer his blessing, then there is nothing wrong with that.
I did think you were a little unkind, but I have to admit I've lost my temper with people on here before (or rather, a person) and made a post I regretted because it was personal, so I'm no better than you have been.
I think all of the people who say "It wouldn't bother me" can't really speak until it's happened to them, with the same history of subtle spite behind it. I would have been upset if it had happened to me.