I think I fall in the same catagory as your sister. Took an asskicker of a course in 1973, certifed in 1974. If I could find my log book I think I'd find about 30 to 40 dives in 74 and 75, then 3 dives in 1990, three in 2006 and now getting back into it.
Breezing through a book reminded me of most of the book stuff: tables, physiology, etc. I found the equipment a lot different: I started with a US Diver two hose regulator and my bcd was more like the plastic pfd you find under your seat on an airliner. There was no such thing as a dive computer. Yet, the equipment part was pretty easy to adapt to, what I'm dealing with are the changes in my physiology.
Intellectually I know I'm 33 years older, but for some reason I still feel like I should be able to drink to the wee hours, get up early, sling twin 100s over my shoulder and hoof it a quarter mile in the sand and not be huffing and puffing. So my biggest issue is adapting the memory of what I used to be able to do at 24 with what I'm really capable of at 57. It's a sobering process.
I'd make sure she gets in the water for a few dives so she get's comfortable with what she can do at T+20.