This is an ongoing series of meditations on faith, in response to readings from the articles that preface The Oxford Study Bible.
I will trust the reader to dig up the context if the meaning is unclear.
When early Christians called one another brother and sister, they weren't just being pious. That particular language of piety didn't exist yet, then.
They talked that way because they were in a culture grounded in kinship, and because converting had in many cases disrupted their own kin relationships. They were building a new framework of kin to fill the gap.
When they used those words, "brother, sister, children of God," they weren't just being spiritual. They meant them.