Alive: When I first started attending meetings, I found the use of brother so and so and sister so and so very much on the 'icky' spectrum. I never really liked it as a form of address, and avoided using it.
Calling people "brother" and "sister" is icky.
It is done to create a false sense of "family" and "trust".
Personally, I find it to be incestuous. It creeps me out when someone says "Oh..Brother so-and-so married Sister so-and-so."
And people wonder why cults encourage incestuous relationships...yeah...go figure. Did you marry your "sister" or your "brother"? Would you?