You are a little safer having booked your transfer through the cruiseline. It only takes about 40 minutes to get from the airport to the port, so you should be fine. Your sisters flight is a bit concerning, but is still doable provided there are no flight delays.
It this sort of worry and stress that has made me decide to always come in the day before a cruise unless it can't possibly be avoided. Yes, there is extra expense of the hotel, but its worth not having the stress before a vacation.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007