My mum (who deserves some sort of medal) used to fly with four of us under ten every year from HK to LHR, and then drive us down to Wales. We caught the night flight, and changed into our pyjamas once the seat-belt light was off. We were also bribed with new toys, which were kept for on the plane, and distracted by drawing pictures/writing postcards for Daddy. People tend to be very good about letting you queue-jump for the loos if you have small children too.
With the jet lag, I always set my watch to the time at the destination as soon as I get on the plane, and try to sleep as little as possible going East, and as much as possible going West; but can imagine this won't be so much help with a little one.
With regard to feeding, if you can at take-off and landing, I believe this helps the ear 'popping' sensation in the same way that sucking on a boiled sweet does, and should help them feel comfortable.
Oh, and be prepared for the fact that Chep Lap Kok have a temperature screening thing you have to walk through - 'tis a result of the bird flu pandemic. They're very friendly though, and it is just a health and safety thing.
Your sister will no doubt know this already, but try and go to Stanley Market, cross the harbour on the Star Ferry and go on the Peak Tram! I really miss HK...
Feel free to ask me any questions; my family still live there! Hope you go and have a fabulous time.