"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."
Interesting trivia: It's never stated outright but if one calculates the relative dates given in the story, Della must still have been in her teens when she had Huey, Dewey and Louie .
(Last chapter happens 17 years after the penultimate one, and Donald was even younger than his nephews' current age when he last met Scrooge. So Della must have been around 16 at the oldest when she had them)