I think anything other than Uncle is fine. He is not their uncle, so that name makes for confusion.
In our extended family, we have Granny, Grandma, Grandma Cabbie, Nanny, Grandad, Grandpa, Pops, Papa, Gran, Grangran, Oma, Opa and various Christian names.
Seems it’s now Harry’s uncle - the Earl of Spencer - having his turn at a “tell all” book.
