My opinion about wedding planning is that whoever is paying for it gets to be involved in the decision-making. For example, if the parents contribute 20 percent of the cost of the wedding, they get to invite 20 percent of the guests.
Weddings are obviously emotional events. At the end of the day, though, it’s their wedding and they can decide how to organise it. If they’re not interested in including you in the decision-making, I’d just roll with it, show up on the day and have a good time. Without knowing the participants, I think that if you put your brother in a position between choosing between his fiancee and you, you’re unfortunately going to lose. It’s a situation best avoided.