My GUESS is it's a mistake, and it was made because he didn't see it as important to be accurate. Every family has it's "genealogist" but in U.S. culture we normally pay as much attention to family trees as some other cultures. Both "cousin" and "uncle" fall in the category of "relatives" (rather than, say, "distant relatives.") He remembered that the Duke has some kind of relationship to the King, couldn't remember for sure, thought it might be "uncle" but was mistaken. He probably didn't bother to check a reference work (more difficult in the pre-Internet days!). Some magazines are famous for fact-checking the articles they publish, but this wouldn't have been done in a novel. P.S. And, of course, it's important to check Wikipedia! I'm going to do it now, but when it is anything important, you want to make sure that the fact really does have a citation. And if it's controversial, you want to check this history and make sure there isn't a dispute in progress! I think Wikipedia is very reliable, but it's reliable _because of a process_ and more people need to understand the process! Do you mean this article? https://en.wikipedia.org/wiki/Prince_Amedeo,_Duke_of_Aosta "... a first cousin, once removed of the King of Italy..." I _think_ the article is probably correct, and I trust it more than a casual mention in passing in a novel--but on inspecting the article and its history I don't see a specific reference source cited for that statement or the family tree. If it were really important I wouldn't accept it without checking elsewhere. As things stand, the reason for believing it's accurate is that "it's said to since 2011, there have been lots of edits, and nobody has changed it." However, for four years it said "a cousin," and it wasn't changed to "first cousin, once removed" until 2011! My judgement: a 90% chance Hemingway just goofed and didn't care, 5% chance Wikipedia is actually wrong, 5% chance of something else...

Not sure what this has to do with the English language, but ... 1. A Farewell to Arms is a work of fiction, it isn't a history of WW1. He could say the Duke of Aosta was George Washington's uncle, and it still wouldn't be a "mistake". 2. The name Aosta is an allusion. Allusions are, by definition, not accidental.