In chapter 17 of The Just Men of Cordova (1917) by Edgar Wallace, the author is describing a conversation between an old uncle and his nephew, who was an assistant-commissioner of police. In a previous passage, the old lord said, revealing the real identity of his nephew:
"I thought it was," said the wounded man quietly. "You had better get on to Southampton. He'll probably pick up Fellowe"—he smiled through his mask—"I suppose I ought to call him Lord Francis Ledborough now. He's a nephew of mine and a sort of a police-commissioner himself. I wired him to follow me. You might pick up his car and go on together. Manfred can stay with me. Take this mask off."
And the mentioned conversation was as such:
Black was hanged at Pentonville gaol on the 27th of March, 19—, and Lord Francis Ledborough, sitting by the side of an invalid uncle's bed, read such meagre descriptions as were given to the press.
"Did you know him, sir?" he asked.
The old earl turned fretfully.
"Know him?" he snarled. "Of course I knew him; he is the only friend of mine that has ever been hanged."
"Where did you meet him?" persisted a sceptical A.C. of Police.
"I never met him," said the old man grimly, "he met me."
So my question is: Was it normal for someone to call his uncle "sir"? And why the conversation was very formal as such, why the author used "a & an", although they know each other??