The Big Island has several nice locations for novice shore diving. In particular, near the town of Kona is a location know as "Place of Refuge". Lots of turtles, lots of fish, reasonably calm, and you do not have to go very deep. One of the great things about this location is that the snorkeling is quite good so you can do your surface intervals while snorkeling to get the most out of your underwater experience.
Even though I have been diving since 1977 with trips in some really increadible places around the world, I find that diving around the Hawaiin islands is still fun and interesting. So much so that I will be diving Oahu for the first time in just over a week to see what she has to offer.
If your sister and boyfriend like to explore topside as well, the Big Island is increadibly diverse with all kinds of hiking, horseback riding, jeep adventures, and an awsome array of beaches ranging from white to pink to green to black sand. The Kona area is very sunny, dry, and warm but lush vegetation and waterfalls are only a short drive away.
Hope they have a great - and safe time in Hawaii, wherever they end up going.
NW_DiverDude