I've had to do three of these speeches so far, with one more to go in a few months... My advice? Have a few drinks and just let it flow!

OK, so maybe that's not the best idea, but toss at least one or two back just to stay loose and calm during your moment in the spotlight.
Brent's suggestions are good ones. Talk about what a wonderful couple they make, and how you've never seen your brother happier before, and how happy you are to have her and her family joining together with yours, and what great people they all are! Yeah, tell a couple cute/funny stories too, but nothing too raunchy and nothing that will embarrass your bro too much (unlike that one beer commercial where the dude spills the beans on all of the grooms former "scores" with the ladies

)...
Good luck, and have fun Chris! Tell us how it went when you get back!