i broke my ankle many years ago while on a trip and i was able to fly home but again that was about 15 years ago. no one said there was any reason to not fly and i do not recall having any problems. if you have it in a cast, then just try to elevate it during some of that time and make sure to ask your doctor if you should be taking one aspirin a day to avoid blood clots. that would be the only concern i would think.

just as a side note, i am actually laid up again because i broke my foot and had surgery so i can empathize with your sil. have a good trip! :)