Well, I hate to be that person, but y'know, there's a lot of back story that we can't see. Like, for example, what kind of person is Fleur? From this brief 28 page story, we can already tell that Fleur is the pining type. The type that really can't let go. What kind of person is the bride? She only had like 4 pages of actual dialogue, so I think it's safe to say we don't really know how she works. But it's pretty obvious that she wanted to help ease Fleur's pain and probably her own. I'm not saying that this wasn't sad and a bit teeth grating, but maybe for the two characters this was really the best scenario.
Like, what if she told Fleur before the wedding and somehow Fleur mustered the courage and balls to crash her wedding with an objection? I mean, I know it's hard for some of us to understand how important family traditions and what not are for some people, but we ought to respect their choices. It would be incredibly disrespectful and bigoted of us to ignore their weighed decisions and make their choices for them.