My friend has an aunt who lives up there and they wouldn't live anywhere else. I have heard that a large portion of the population is retirees from England. I think the Orkney's are beautiful! I don't really know much about living there but-these are things I have heard and observed (I'd live up there if I could!).
Kirkwall is a pretty substantial size town, well compared to the surrounding areas, and they now have a Tescos and there is a Lidl there up there. There are lots of planes and ferries that go in between the islands and there are regular ferries between the islands and Scotland and you get a pass for the ferries for living up there. They have a hospital, GPs, ambulence service, etc. They also have excellent schools (not that I suppose that matters to your inlaws).
Off the islands, nearest town is Wick, which is a pretty small town, but it does have an airport and other such necessitates. Its only a 3 hour drive or so to Inverness too, and from here, although its not a great airport, you can connect enough to other places. Down here we have all the usual shops, etc and there are a lot of people who come down here to Inverness to shop for things and make the journey back up. There are twice a day buses up that way too.
The other thing your inlaws may want to know is that I guess its a really, long, long winter. Like only 4 hours of daylight and wet and damp the entire time.
I would guess that things are more expensive, because of the travel cost involved with moving them up there. But they have good local beef and veg and beer too.
No trees, well very few anyways, on the islands -the winds can get up to hurricane force on a pretty regular basis.
I know others on UKY actually live there, so they can give a lot better advice than I can, but I think there is definitely a laid back way of life up there and absolutely gorgeous and rural.