How do I "stop this problem before it happens"?

My older brother, Daniel, has finally proposed to this wonderful girl he's been dating for about three years! Me and my younger sister, Bethany, were ecstatic to hear this since everyone in my family adores her. The problem is she doesn't have any sisters, and her closest friends are me and Beth. I'm worried that when it comes time to pick the maid of honor (which is soon, they plan to get married this summer since they are so in love) :) ... We might have some, problems... My sister gets extremely jealous, and so do I, so how can I, "stop the problem before it happens"?