Well, there is an official comic detailing what happened to Matilda -- she moved back to Scotland and became the official caretaker of Castle McDuck. Maybe I'll post some scans of that sometime. As for the rest of them, doubtful.
Don Rosa did ponder a comic in which Huey, Dewey and Louie tried to find out what happened to their long-lost parents, but never actually made it because, as he said, he couldn't find a satisfying way to end it. If they found their parents, then it would be unrealistic for them to go on living with Donald, and there's no way the Disney status quo would accept that they suddenly didn't live with their uncle anymore. If they found them and for some reason discovered they'd be better off with Donald, it'd make for a fairly weird story and the censors probably wouldn't allow it either. If they found their parents were dead, that would make for a bleak and depressing ending that didn't fit the comic. And if they didn't find their parents at all, it'd be a pretty pointless story.
That said... when I said it was uncertain what happened to Della Duck, Donald's sister and HD&L's mother, I meant officially. I actually happen to know a bit more about the subject, but I didn't mention it in the post itself because it didn't really have anything to do with the story. But, I'll tell it here:
Back in 1994, Carl Barks (who was 93 at the time) was on a tour of Europe and among other places came to Norway for a meeting with the fans, receiving the "Sproing" award (annual Norwegian award for best comic) and an informal Q&A. As a junior member of the Donaldists, I was at this meeting (I was fourteen at the time), and therefore I was present to hear the answer when Barks was asked by one of the fans what happened to Della Duck.
So -- this answer comes directly from the creator of Duckburg himself: Della Duck ran off. She was so tired of raising those kids that she simply shipped them off to Donald, left Duckburg, and never returned. Possibly she's off globetrotting the same way Scrooge did in his younger days?