What is the first occurrence in Church history of addressing a religious male or female (i.e., one who has taken vows of obedience, chastity, and poverty) with "brother" or "sister," respectively?
It may be possible that St. Benedict of Nursia (480-547) in his Holy Rule is the first recorded instance of maintaining the fact that his religious should not address one another simply by their names.
Chapter 63 states the following:
Let the junior brethren reverence their seniors, and the seniors love their juniors.
In calling each other by name, let no one address another by his simple name alone; but let the seniors call the juniors Brothers, and the let the juniors call their seniors Fathers, by which is understood paternal reverence. But he Abbot, since he is looked upon as representing Christ, be called Lord and Abbot; not that he has taken it to himself, but for the honor and love of Christ. He himself is so to consider it, and so to act as to be worthy of such a dignity.
Wherever the brethren meet one another, let the junior ask a blessing from the senior. When the senior passes by, let the junior rise and give him place to be seated; nor let the junior presume to sit down unless the senior bid him do so, fulfilling thereby what is written: "With honor anticipating one another." - CHAPTER 63 of the Rule of St. Benedict.