Does your brother live in Key West, or is this a destination wedding? I'd be more inclined to go if he lived there, than if he simply chose his ideal wedding location and assumed all his friends and family would be willing to shell out the bucks to go.
If you can't afford it, don't go. Invite him and his new bride to come visit you.
If you can't afford it, don't go. Invite him and his new bride to come visit you.