I noticed this was common among various cultural groups. It was kind of a signal to the children that this other adult was screened by the parent and accepted as trustworthy. Of course, it's not much of a stretch to realize that someone with ulterior motives could patiently wait out the seduction phase of a "friendship", only to eventually abuse the trust that was given to them.
This is why I have a problem with the terms Brother™ and Sister™. It implies that the individual(s), as bona fide member(s) of the Christian Congregation of Jehovah's Witnesses™, have met some kind of standard for exemplary conduct. However, even when one of these Brothers™ or Sisters™ is found out to be a pedophile, and even if the Congregation Elders™ DF the person... what happens when they get Reinstated™? They get to be called Brother™ So-and-so or Sister™ So-and-so again, as though nothing ever happened.
The WTS needs to stop doing this, because if they aren't going to allow families to protect themselves with fair warning, they'd damn well better find a way to do it "discreetly". Why not disallow them from referring to themselves as Brother™ or Sister™ all together? Why not just refer to these people as Mr. or Mrs.? It would be such a simple, but meaningful distinction...