Not blood makes family, not matching DNA makes family, no legal definitions make family. Family is the feeling. And these Nuns, Ducks, Priest, other animals, guardian angels and stuffed animals have that feeling for each other.
With this long runner and popularity, I wouldn't be surprised this gets animated sooner than later.
While I'd absolutely love a Little Nuns anime (even if it was just short 5-min episodes), I have doubts if that would be possible. The director, animators etc. would have to largely expand on the scenes.
