And they all called Scrooge McDuck "Uncle Scrooge". There was an investigator tried to get to the bottom of this conundrum years ago. He worked Donald over quite thoroughly until he broke down in tears, whining:

"They're my nephews!"
*whack*
"They're my brothers!"
*whack*
"They're my sons!"
*whack*
"They're my nephews!"
*whack*
SOB

... and so on.

(At least that's what they thought he was saying. Nobody could really understand him anyway and all the tears and snot flying around just made things worse)

Forget it, Jake. It's Duckburg...