It is not very sensible to put your email address in an open web forum like this. I now know your surname, your town and your email - identity theft is easy.......
Secondly Transatlantic cruises ususally happen in April and November. So I suggest your relations fly at least one way. Transatlatic cruises do not end up in Seattle. On a positive note - they could take the Brilliance of the Seas out of Miami on April 14th 08 and this actually stops in Falmouth, so with permission they could disembark there.
The cruise would have to end or begin in that port expect for Costs where you can join in the get off in several places and it is really good because then you do not get the big queues and mayhem on the first day.
Why don't you cousins do a cruise next spring when the cruises are moving back to the med and then it is one way , I am sure that some of them do them in the fall as well for Seattle.