As an only child, I don't know the rules when it comes to siblings. But I would imagine that there is a degree of loyalty expected from brothers and sisters. And I would expect my hypothetical siblings to prioritize me over my exes. And if they didn't, well, I'd throw a hypothetical fit.
But here's a non-hypothetical scenario:
The woman explains that her brother invited her soon-to-be ex husband before they knew about the cheating.
My (34f) brother (31m) is getting married in a month. My brother is not friends with my friends but he knows them by association and gets along with them and he's invited them to his wedding. My soon to be ex is also invited. He was invited anyway before I found out certain stuff.
Her (soon-to-be-ex) husband and brother have known each other since high school. But she says her (and her husband's) friends only know her brother by association.