In need of ideas/suggestions of how to go about preparing a speech for my brother's wedding ceremony?

Hi All-
My younger brother and only sibling is getting married. He and his wife-to-be have asked me to prepare a speech of my choice for the ceremony, which will be given before vows are exchanged. Prior to asking this question I checked out some videos of wedding speeches on youtube to hopefully get some ideas. I realized that just about all of these speeches were done post-ceremony at the reception, which I thought were much more relaxed and less formal than the one I need to prepare.
I feel a bit stuck and would really appreciate some ideas to get me started on a wonderful speech. Thanks so much!