According to Don Rosa, he is!
Don Rosa described Daisy as the sister of Donald’s brother-in-law, so Della’s husband and the father of Huey, Dewey, and Louie is/was Daisy Duck’s brother.
So far, Huey, Dewey, and Louie’s father has never been given a first name. His last name is only listed as “Duck,” in the Carl Barks and Don Rosa family trees.
Don Rosa explained that the surname “Duck,” is very common and the equivalent of “Smith.” All the same, it would still be quite the coincidence for both Donald and Della to fall in love with people with the last name Duck… but less so if their loves were related to each other.
When I learned that HDL’s father and Daisy could be related and asked the good people of DuckTalks if they’d ever covered that topic as a possibility for DuckTales 2017, Infinirama said he had early on, back on October 20th, 2017 (his post can be viewed here). Nevertheless, he’s given me his blessing to bring this matter to the fandom’s attention once again.
Frank Angones has said on Tumblr that he and the Ducktales 2017 crew are very familiar with the Duck family tree, and that they are primarily working from Don Rosa’s family tree for the show.
We also know Daisy is set to make an appearance. Frank has answered several questions on Tumblr from fans wondering if Daisy will be in the show. He answered one by saying, “spoilers,” and at least four with “Please see pinned tweet.”
In June 2018, he also answered another question from a fan and explained some of their early plans for Daisy and why they decided to scrap those plans.
Then last March, we got a hint that they may have figured out who they want their Daisy to be:
This could mean that Daisy is set to appear either toward the end of Season 2, or sometime during Season 3.
What’s interesting is that if the DT17 crew are sticking to Don Rosa’s account of the Duck family, then Daisy might truly be Huey, Dewey, and Louie’s aunt on their father’s side in the show. And if that’s the case, we can expect her appearance to shed some light on HDL’s father’s identity, whereabouts, and/or fate.
Now that Della is home, many fans are expecting some little hint about the father – any tiny bit of information that could possibly indicate that HDL’s eggs weren’t just dropped off by a stork on the doorstep of McDuck Manor one morning with a note that said: “To Della Duck.”
After all, when asked about HDL’s father last June, Frank did say “one parental mystery at a time.”
With the Della mystery nearly wrapped up, it may almost be time to move on to the next parental mystery!