A nice dress that will be warm enough for you, depending on the weather there. Fall colors or winter colors are best this time of year. You could also wear a dressy type pantsuit too. Check around to see what the rest of the family thinks. a lot depends on the religion too.
Depends does your brother or his bride have a color themed pick out, when i got married i had a light blue for mine and Mine was also outdoors, I even had light blue in my dress, find out from your brother what the color themes are and if there is none try something that will match the flowers of her bouquet or the flowers in the wedding.