The brides were hurt by the women’s attempts to make them look foolish. Which is the only explanation for their actions, knowing the women as I do (and you don’t, I presume. You could be one of them, for all I know). Whether those attempts were successful, is irrelevant.
The two women (MIL and SIL, separate families) tried to humiliate the brides and get a rise out of them by wearing white. It was also them clearly letting everyone know their thoughts on the women chosen by their son and brother. (The SIL hated ANY girl the brother saw. She became deeply territorial about her brother, particularly when pissed, and it used to make his skin crawl as it was so weird). Sadly, the grooms felt the most humiliated, by their mum and sister’s public display of not liking their wives.
The MIL accompanied her actions by slagging the bride off in the toilet to anyone who would listen.
So in my case, both times I witnessed the ‘wearing white’, it was spiteful and designed to hurt a bride, even if the outcome was a hurt groom and the wear-ee being the one that looked a twat.