As someone who had to fight literally every step along the way with many mental case mexican catholics about how I wanted my wedding to be...tell your brother to hold the line, it's his day too, not theirs.
We got a lot of no shows on the wife's side because I insisted in it being on a boat instead of in a church, insisted on a non catholic ceremony (our priest was a canadian lesbian, she was great) and a smallish reception in Vallarta instead of a big mexican reception in Guadalajara. Wife was tough to get on board at the beginning, but got excited when we started viewing the boats.
The people we gave a proper fuck about mostly showed, and almost 3 yrs later wife is entirely happy with how things went down.
Hold the line.