Yes, we are only going to rent one car. We will probably arrive one day early and then pick them up at the airport. You think Providence would be better than Logan?
Are you brother and niece definitely flying into Boston or can they re-route to PVD (though they might have better options flying into BOS anyway).
It's definitely easier to get in and out of Providence (by the way, the airport is not in Providence but several miles south in Warwick). From my experience, it's now easier to get to Logan with the new tunnels, but can still be tricky getting out.
If they already have airfare from the West coast into BOS, one suggestion I can make is that you stick with your original plans to pick them up in Boston, but instead of driving to Logan, drive to the outlying T-Stop in Quincy, and take the 'T' (subway) to/from Logan
http://www.mbta.com. From Quincy, it is a straight-shot down Rt 3 to the Cape.
I have done this in the past just to avoid the hassles of driving around Logan.
Of course this will be a tad more difficult if there is a lot of luggage involved and might add a few dollars to the cost (parking in Quincy and fares for the 'T')
FYI, I do not know which, if any, car rental companies are located at the Providence Train Station.
Everything always takes us at least half an hour more than Mapquest says it will.