Cheesecloth. Or Gauze.
In the interest of avoiding anything cumbersome, hot, expensive, itchy, complicated or difficult to maneuver, the field of potential costumes has been narrowed. I'm still thinking mummy. (Which would also give me the satisfaction of getting some extra mileage out of that $29.95 inflatable sarcophagus.) Or maybe my vintage Burberry trenchcoat, Wayfarers and gauze - you know, the Invisible Woman. It's obvious I'm psyched about seeing my sister, but not about the costume thing.