I agree with the stay awake to HKG part.
Two ideas to ponder: first, flip the seats (at least your Mother's) periodically to HKG, even if you are awake - it will probably lessen the swelling to get your legs up. Falling asleep? Put it back in the seat config. Just get your feet up at least half of the time. Second, you will be off the plane at HKG, so take advantage of it, get your blood moving and walk around.
PACK A PAIR OF LARGER SHOES in your carry-on (preferred) or on the top layer of your checked bag (at least a half-size). Most crew pack one pair of larger shoes to wear in-flight. Works great for me, too. Your feet will thank you! Lightweight/casual preferred; you could always take a pair of sandals and use those.
I cannot stress this enough if your Mother has never flown a long-haul before. I have seen people in abject misery crawling off the plane in SYD because they drank booze and/or didn't drink water, or didn't change their position during the long flight. Y class seems to be the worst, but you can get it in J as well. Ask any shoe shop in SYD, and they will tell you some stories of people having their feet swell 3 full sizes! Don't be one of them. Drink, walk, move, and get your legs up as much as possible. Use the Y lavs (if you want to move about in the cabin). Makes you appreciate what you pay for J.
Melatonin, Valerian, and homeopathics are out there for sleeping and combating Jet Lag. One of them, No Jet Lag, I have personally used and I'm pleased with the results. You can get it many places.
Enjoy SYD! 3 days are just enough to make you want to go back! Or never leave...
