Autie-J

Because it's weird that Donald wouldn't fulfill his thought to be dead sisters last wishes even if the names are a bit weird. Unless you look back in Season 2's Last Christmas. Donald clearly knew at the end that Dewey was Bluey, the family members from the future that visited him and Della from the future that one time.

Probably soon after Della disappeared I bet, after looking back on how Dewey was so emotional to child Bella, Donald probably guessed that future family member was his own nephew. But the question remains.

Which one?

Donald had three eggs to look over, and he didn't know which one of the three eggs would be the kid from the future. Donald didn't really have much to go on. He was a kid when he met the time traveler and only knew that their name was something -uey.

Not wanting to break the time line, when the triplets were born Donald just gave them all similar sounding name just to be safe. Thus we get Huey, Dewey and Louie.

Six kids... till now

I know Webby is Scrooge’s girl, but she's included in Donald’s family hugs so I may add her too

"so their default form is-"

It does sort of feel that way at times, particularly when it came to Sasha and Marcy in season 3A.

Like I've made clear before: I understand what the writing was going for in trying to portray Anne suppressing her emotions about what happened with Sasha and especially Marcy in True Colors so she didn't have a complete breakdown. But to me, the show swung a bit too far into Anne's suppression, to the point where it came across as her barely caring that her two best friends could be dead.