Most airlines require a certificate signed by a doctor stating that the passenger is fit to travel and does not require any special care when there is an obvious medical condition. If special care is required, most airlines are quite happy to sell it to you (my father traveled once in somewhat similar circumstances and was required to buy 6 full Y tickets and travel with a nurse; in return the airplane was configured with a hospital type bed that took the space of 6 Y seats and was curtained off from the main cabin). You may wish to check with CX to determine what certification they require, if any, before allowing your sister to travel (it sounds like she is well enough to travel as a normal pax).