Something what lives in my mind with free rent for some years ago is the conclusion that ALL of The Sensational Six have siblings, because all of them has showed nephews/nieces at least one time. You know, Mr. Disney made the familiar relationships between uncles/aunts-nephews/nieces, grandmas/grandpas-grandsons/granddaughters and between cousins and siblings, and the characters never get married or have their own kids. So, all of the main characters have siblings.
Mickey have the twins Morty and Ferdie Fieldmouse;
Minnie have the twins Millie and Melody (principally);
Donald have the triplets Huey, Dewey and Louie;
Daisy have the triplets April, May and June;
Goofy have Gilbert Goof.
As we already know about Della Duck, Donald's sister in dt17 and in some dutch comics and some people know about Felicity Fieldmouse/Amely Mouse-Fieldmouse/Amalia Fieldmouse, Mickey's older sister in some comics, we never know or talk about the siblings from the others characters. I think, if it's uncommon to see their nieces/nephews, it means they lives with their parents, and sometimes they goes to the aunts'/uncles' houses. Or maybe they live with their uncles/aunts because they doesn't have parents too. Or even they live with some grandma/grandpa and visits their uncles/aunts, as in some italian comics kid Donald lives with his grandma and used to visit uncle Scrooge, or all of them lives together (Rota comics).
Pluto is the only character who had kids with Fifi the Peke in an old cartoon (Pluto's Quin-puplets, where one of them was called Pluto Junior). Instead of having a nephew, he has a younger brother showed in the cartoon Pluto's Kid Brother.
Minnie have a sister called Mandie/Mandy Mouse, who is a naughty mischievous lady, who could be Millie and Melody's mother, as in the show Mickey and the Roadster Racers, Minnie says something about her sister to her nieces, they says something and Minnie answers: "Your mom is right!", or anything like that.
Daisy have a sister (who is AMJ's mother) what my source tell she doesn't have a name, but I remember saw a comic (srry but i don't know the name) where Donald says her name is Violet (maybe it's just a joke from him, but who really knows).
From this post:
Goofy have a brother called Gaffy (Pappo in the original Italian version), who disappeared in the jungle and was reunited with his brother in the Mickey Mouse comic The Quest for Tarzan, and maybe he is Gilbert's father.
It would be great if we could know more and more accurately about the families of the "Sensational Six", since everything I could find out about their siblings I put here. Maybe I'll make other posts about families later on, this one I had in mind for a long time.