Just let him be himself! Just make sure that you sister understands the "charms" of a three year old, and does not have specific expectations of a poised and perfect little gentleman. I'm not saying to totally let him run wild - you will need to prepar him, and let him know what is expected. But if he dilly-dallys down the aisle (how the heck to you spell that?:), or runs like a crazy man - I think that just adds to the fun, and everyone needs to be understanding of that. If your sisters not up for a relaxed time that everyone can enjoy, then maybe he is too young. But if she's glad to have her nephew be a part of her big day - then she's got to be ready for some three year old entertainment!
Best of luck - and I hope it's a wonderful day for everyone!
Sera
So, I can’t exactly say what happened at the wedding yet, but I can say that my sister, and my whole family for that matter, is a comedic bunch of people, so I’m sure no matter what goes down, it will be taken lightly.