The one bit of advice I can give you is be prepared for ungodly heat and humidity!
September in N.O. is the height of hurricane season and is one of the hottest months in the South!
Pack lots of clothes to change into, and plan for the wedding to be indoors! This is coming from a born and raised Southerner, so I have a feeling that anyone north (especially Canada, lol) will feel the heat a whole lot more!
Congratulations, by the way, for your cousin! Best Wishes for her marriage!
P.S.--N.O. is a FUN town, so just enjoy!