Thread:Hey1234/@comment-25090039-20161211114346/@comment-25090039-20161213154817

Obviously I can't answer why its authors decided to make Ludwig Grandma Duck's cousin all those years ago. However, the fact remains that they did include it in an official Disney production.

As already mentioned (with source) in Ludwig's article on the wiki, Ludwig was actually said to be "sort of an uncle" to Donald (i.e. an elderly relative loosely referred to as "uncle"). He is not a "full" uncle of Donald's anyway, since he's not Grandma Duck's son and not Scrooge's brother.