Gladstone has been referred to has HDL's cousin (Gladstone Returns).
Technically, Gladstone *is* HD&L's cousin, since he's the cousin of their parent (making him their first cousin, once removed). I no longer accept the Rosa version that Fethry is Della's first cousin, but if he were, then HD&L would be right to call Fethry "cousin" as well.
Somehow, I can't remember ever assuming, as a child, that Duckworld stories were set in my home country. I was told early on that the Walt Disney Company was American, so I saw all the characters linked to it as... well, American.
Funny anecdote: I did read a few Dutch stories that involve Sinterklaas, but it doesn't bother me much. My theory is that the first generation of Duckburgians included a large amount of Dutch colonists, which explains certain elements of Duckburg's unique culture (not just Sinterklaas, but also that Holland festival that appeared in one story by Carl Barks).
It's consistent either with their being cousins or being siblings, but at any rate, this tree definitely presents Grandma and Scrooge as belonging to the same branch of the family tree, jus like Gladstone and Blackduck do to their own single branch in a way that indicates they're more closely related. Funny that this is the same year as the first original Italian source to definitely describe Scrooge and Grandma as sibling, although it clearly must be a coincidence.
It's also worth noting that in 1953, another Fallberg-written story, Family Tree (that's the story with Pokerface McDuck) had featured an even smaller, even less detailed Duck Family Tree.
On its own I wouldn't necessarily read that as showing Scrooge and Grandma to be siblings, but it's easy to see the 1960 family tree as an evolution of this one; in both, Scrooge and Grandma are on a fork at the top of the tree, though it's only the 1960 one that achieves this in a way that clearly spells out that they're related by blood. The 1953 story also interestingly sandwiches Grandma between Scrooge and Pokerface, two McDucks, so there's that.
Let me try to sum up what's been said on this thread:
First German story dialogue with Scrooge and GD as siblings: 1958 translation of 1956 "Scrooge's Second Childhood" (reprinted same way in Germany in 1997)
various German translations through the years state that GD is Donald's aunt (and Daisy's aunt, sigh) *and* once, at least, HDL's [great-?]aunt! (She refers to them as "meine drei Neffen." I can understand the theory that Donald is her nephew and HDL are her grandchildren, but making HDL her [great-]nephews seems bizarre. Have Germans in general thought of Donald as GD's nephew and HDL as her grandchildren? Then this story is an outlier. How did Grandma Duck get the "official nickname" Grandma Duck if she is nobody's grandmother?) Drakeborough's German correspondent says that the theory that GD is Donald's aunt is not universally accepted among German translators today. (Well, they couldn't pull that off in translations of L&T, anyway.)
Italy: (not implying that these are *all* the stories that showed Scrooge & GD as siblings, just listing ones mentioned in this thread)
1960 poetry by Martina/painting by Scarpa (but 1955 story by Martina shows them growing up in the same town/school, but definitely not siblings)
1973 story ("totem decapitano") by Martina, dialogue seems to have been edited to make Scrooge & GD brother & sister
1984 Rota's "from egg to duck" depicts Scrooge and GD as siblings, GD as Donald's adoptive mother--later Donald found to be actually Scrooge's nephew?
1996 story "le notizie...fraterne" shows Scrooge, Gideon and GD together as child-siblings in Scotland
last story known by drakeborough to have Scrooge and GD be siblings is 1999 "un cent per Millicent"
1969 "Circle of Evil"--in the Italian translation, GD calls Scrooge "fratello caro"
1982 "No Sale" and 1983 "Good Old Exciting Days" by Nofziger--in German translation, it is said, they are presented as siblings, but in Italian translation they are not
random American stories:
1953 Vacation Parade 4 Grandma Duck story by Christensen--GD mentions "Cousin Scrooge," while Donald is referred to as GD's nephew (but then, this story involves Minnie Mouse, so... Is this the story Deb referred to, where Minnie calls her "Grandma"?)
1963 "Too Much Mush" author unknown--Donald referred to as GD's nephew (thus, possibly, GD and Scrooge are siblings)
A 1994 "photo album" piece shows the family with baby Scrooge, child GD and their parents. Rob reports that his impression is that Scrooge and GD were originally said to be Donald's uncle and grandmother in Dutch comics, and not portrayed as siblings in Dutch comics until the late 1980's or 1990's. Can any other of you Dutch folks confirm this? (Of course, they could be Donald's great-uncle and grandmother and still be siblings...but Rob doesn't remember their being presented as siblings earlier.) Rob says that the Dutch may have gotten the sibling idea from the Italians or the Germans. Currently, though, it seems that the sibling theory is working canon for the Dutch editors. See Scroogerello's link to Geradts' blog, where he says a story of his was rejected in 2014 because Scrooge was too nasty to GD, and he wouldn't treat his sister that way. Also, a brand-new Dutch story has Scrooge calling GD "dear sister".
About Italian stories, there is a typo in the 1973 entry: it's not "totem decapitano", but "totem decapitato".
Another I-coded story in which they are sibiling is this 1971 frame story, written by Gian Giacomo Dalmasso and drawn by Giuseppe Perego. I have also heard that Scrooge and Grandma are said to be sibilings in the 1966 Italian story Nonna Papera e l'aratro all'antica, written by the Barosso brothers and drawn by Giuseppe Perego. However, I have't verified this yet.
As for the 1963 American story "Too Much Mush", we saw on page 2 of this thread that Donald is referred to as GD's nephew, but in the same post Scroogerello wrote "I believe that, in that same story, Myron also specifically refers to GD as Donald's grandmother". Can this be confirmed?
Scrooge MacDuck : I think that in both stories the artist simply put in the tree as many of Donald's known relatives as possible (including for some reason even Daisy and Gyro) and just placed them at random, without bothering to decide how these characters were related.
I'll post another version of the panel from the second story since the current version is a bit hard to read:
I admit I haven't read it yet, but the point I was trying to make is that we shouldn't read too much by the position of the trees in which the characters are placed, since in both cases it seems to be random.