I'm sorry, the CM's at AirTran aren't meteorolgists and it's not their job to predict whether or not the flight will be cancelled or not. I think this falls squarely on your sister and her friend.
As for AirTran itself, fees or no fees, every flight I have ever booked on AirTran from PHL to MCO has ALWAYS been cheaper than SW or USAir. That includes the fees. And although I have had minor ajustments in a couple of my flight times, their customer service has always been first class. They have allowed me to book a different flight and a different airport on 2 separate occassions.
My only complaint with AirTran is that they will soon be taken over by SW. Yeah, I won't pay "fees" but my fare will likely be higher that I could have gotten on AirTran.