I agree with pp that AKV is unique & offers what no other DVC on property has But you know your niece & sis inlaw best; would the novelty of it wear off quickly for them ? They may prefer BCV for Stormalong Bay (if they don't have a similar type resort back where they live. I like BWV/BCV resorts location because it is convenient to go to other resorts,DHS,Epcot. I'd say VWL if you think they'd be wowed by the Lodge & biking/walking to Ft.Wilderness and BLT if modern and resort monorail hopping would be their thing.
So hard to decide, so many great reasons to stay at all DVCs Depending upon how many days the trip will be, I'd think about a split stay.
Have fun planning, it's sure to be a magical trip for them
Have you considered a split stay at different DVC properties? Depending on how long your relatives will be here, that may be a great way to sample different Disney experiences without too much inconvenience. A night or two at BLT while experiencing MK, a few nights at BWV/BCV will experiencing EPCOT and DHS, and a night or two at AKL at the end for Animal Kingdom. Don't forget to mix in the water parks, either!
If I had to pick just one resort, I agree that AKL is a great choice. Although we are biased because it is our home resort, I think if this will be their first experience at WDW, it is the best escape from the hustle and bustle of the parks. Quiet, relaxing, very peaceful, and a great benefit to have the extra bathroom in the 1BR and 2BR units!
There really are not any bad choices among the DVC properties. I'm sure they will have a great visit no matter where you stay!