In the rural Southern US, children were not to call adults by their first name, JW religion or not. Anyone not well known as always Mister so-and-so, or Miss/Mrs so-and-so. Adults close to the family would be called Uncle or Aunt, even if not really related. Like your father's best friend would be Uncle Bob.
In the JW religion, all adults were called Brother or Sister (followed by last name) by children at all times. There were some people I never knew their first names growing up.
As for adults, the use of Brother/Sister was usually confined to JW settings - Kingdom Halls, field service, assemblies, and usually with 'friends' that were not in your close circle of friends. But it was used enough even in some situations that sometimes it made adult JWs stick out in public when they referred to another adult as Brother or Sister.