are your FI's parents contributing financially to the cost of the photographer or the dress?
If not, politely tell them, "I realize you may find it foolish or a waste of money, but [FI name] and I have decided that we are going to do this. It's our wedding and this is special to us. "
If they are contributing, it's time to have a different kind of chat -- Thank them for their financial contributions but politely explain that the fact that they gave you money for the wedding does not mean that they get to make any of the wedding decisions.
Of course say it all with a smile, and follow it with, "I know you guys understand, right?"