How would you deal with an over-dramatic bride?

Although her wedding isn't until October, my sister in law is already a Bridezilla. She told my brother in law's wife that she didn't care how much money the "other people" had to spend because we'd "do it or not be in her wedding." Brother in law's kids are also supposed to be in the wedding. The bride has chosen very expensive clothing and she will not be swayed. The exact same things can be found at a more agreeable price, at other stores. Yes, I'm talking about renting them, but the price of the rentals are based on the price of items. I agreed to have my two oldest sons in the wedding at her request, but I was expecting to coordinate with her to chose the clothe- so she would get what she wanted and I could afford it. Now anyone who has balked at the cost of her choices is going to "ruin her wedding." I don't want to cause her distress, but she's causing the rest of her family to want to run away screaming! Because of her recent comments and attitude, I'm tempted to tell her that my kids will not be in her wedding. How would you tactfully address this issue if it were your family?