Tried to formulate a question as neutral as possible to avoid spoilers about CC, but here they come! Leave now if you haven't read/seen/finished it ;)
So my actual question is rather:
Why didn't anyone find it weird that Amos Diggory had a niece?
I mean, the British wizarding world is kind of small. All over the books, we can see that most people are related or friends or friends of relatives, or neighbours of relatives or colleagues or something.
The result is that almost everybody knows (at least has heard of) everybody.
“The pure-blood families are all interrelated,” said Sirius.
(Order of The Phoenix, Chapter 6, The noble and most ancient house of Black)
Even baby Malfoy had heard of the Weasleys, even if their family are not really friends:
No need to ask who you are. My father told me all the Weasleys have red hair, freckles, and more children than they can afford.
(Philosopher's Stone, Chapter 6, The Journey from Platform nine and three quarters)
For instance, the Weasley's : Arthur and Molly knew very well Amos, since he lived not far away from them and worked with Arthur:
"Amos!" said Mr. Weasley, smiling as he strode over to the man who had shouted. The rest of them followed.
Mr. Weasley was shaking hands with a ruddy-faced wizard with a scrubby brown beard, who was holding a moldy-looking old boot in his other hand.
"This is Amos Diggory, everyone," said Mr. Weasley. "He works for the Department for the Regulation and Control of Magical Creatures. And I think you know his son, Cedric?"
(Goblet of fire, Chapter 6, The portkey)
Mr. Diggory’s head looked around at Mrs. Weasley.
“Sorry about this, Molly,” it said, more calmly, “bothering you so early and everything…but Arthur’s the only one who can get Mad-Eye off, and Mad-Eye’s supposed to be starting his new job today. Why he had to choose last night…” “Never mind, Amos,” said Mrs. Weasley. “Sure you won’t have a bit of toast or anything before you go?”
“Oh go on, then,” said Mr. Diggory.
Mrs. Weasley took a piece of buttered toast from a stack on the kitchen table, put it into the fire tongs, and transferred it into Mr. Diggory’s mouth.
(Goblet of fire, Chapter 11, Aboard the Hogwarts Express)
Surely they would have known Amos and his wife were only child.
In such a village atmosphere, how many children did Cedric's grand-parents have had was probably public information.
So how comes no one ever wondered how Amos could have a niece?
The Weasleys (Ron or Ginny), the people working at St Oswald’s Home for Old Witches and Wizards, the other residents of the hospice, the people visiting them…
Amos himself was Confunded alright, but how comes no one ever went “Hello Amos, how are you today? You’re saying this young person is your niece? How is this possible, I thought you didn't had any sibling?”