How it works in my house:
"Son, do your chores please."
"Yes, sir."
Done.
replies(5):
"Son, do your chores please."
"Yes, sir."
Done.
It is plainly offensive to address kin as "sir".
You have no evidence that the GP abuses his children, or even that his children literally call him 'sir' (he may have been exaggerating for effect).