Donald Duck apparently has a sister. Officially (at least now) she is called Della Thelma Duck, but she has also been referred to just as Thelma. She was originally called Dumbella in the 1938 short that introduced the boys.
According to one Disney comic she's Donald's twin sister.
Scrooge goes around the world making all kinds of business deals, notably including forcing the Voodoo tribe of sorcerer Foola Zoola off their land. In revenge, the sorcerer curses him to be constantly haunted by Bombie the Zombie. He returns to Duckburg and first meets his ten-year-old nephew Donald Duck, along with his twin sister Della Thelma Duck.
Their father has been left as an intentional hole. There are no canon mentions of him anywhere.
Note he's not even depicted here in this family photo from a 2014 Dutch comic that depicts Della as stuck in a time warp in space
There are hints that the Disney XD reboot may tackle this
But beneath the show’s adorable, big-eyed facade lurks a slew of disconcerting questions, including: Where are these kids’ parents?
“It is something we’ve thought about,” Ben Schwartz, who voices middle brother Dewey Duck, admits to TVLine. “And that question is something the nephews are inquiring about as well. … The characters do wonder where they came from, and they’re very into exploring that.”