What brethren actually means
Brethren is an archaic English plural of brother. It renders the Greek adelphoi, which named siblings of either sex when applied to a group. A first-century letter addressed to the adelphoi of a city was addressed to the women in the room as well as the men, which is why this translation prints "brothers and sisters".
Where the word comes from
Adelphos is built from a copulative prefix and delphys, the word for womb. It names people who came from the same one. Greek then used the masculine plural for any mixed group, the way certain English speakers use "guys" for a table with women at it, so adelphoi could mean brothers, or brothers and sisters, or a room of unrelated people being addressed as family.
The wider use was ordinary. Trade guilds and religious associations in the Greek cities called their members adelphoi. Private letters recovered from Egypt use it for people who share no bloodline at all. So a listener in Corinth hearing the word read out at the top of a letter recognized kinship vocabulary being applied on purpose to people who were not kin.
In the gatherings that vocabulary had teeth. These households mixed slaves with the people who legally owned them. Paul asks Philemon to take Onesimus back as a beloved brother, which in a Roman household meant rearranging who counted as what. James 2:1 puts the address and the demand in one breath: "My brothers and sisters, as people who trust in our glorious Master Jesus, The One, don't play favorites."
Middle English carried both brethren and brothers as plurals of the same noun, with no division of labor between them. By the time the King James translators were working, brothers was settling in for blood siblings while brethren was being kept for the members of a company, an order, or a trade. The 1611 wording was already slightly antique when it was printed, and it has not moved since.
Why "brethren" gets in the way
Nobody says brethren. It survives as part of a few denominational names and in dialogue written to sound old. Dropped into a letter from Paul, it makes a note between friends read like the minutes of a lodge meeting, formal where the Greek was warm.
The heavier cost is who the reader pictures. English brothers, and brethren behind it, produce a room of men. Romans 16 names Phoebe, Prisca, Mary, Junia, Tryphaena, Tryphosa, Persis, Julia, and the mother of Rufus. That is the letter's own account of who was there. Printing brethren where the Greek says adelphoi conceals half the room from a reader who has no access to the Greek.
Adelphos and its plural run to roughly 340 occurrences across the New Testament, and the effect compounds over all of them. Each one is a small nudge toward an all-male picture of the early gatherings, and none of them announces itself as an interpretive move, because it looks like a plain literal rendering.
What this translation prints instead
The NGET prints "brothers and sisters" wherever adelphoi is used as a form of address. Galatians 3:15 reads "Brothers and sisters, let me use an everyday example." 1 Corinthians 10:1 reads "I don't want you to miss this, brothers and sisters." The reasoning is that the Greek word covered both, and English brothers alone does not, so the longer phrase is what keeps the sense the same.
The standard objection is that this adds words the Greek does not have. The reply is that word count is not the measure. A rendering that covers only men has changed who is being spoken to, and that is a larger alteration than adding three syllables. Where a passage is discussing literal male siblings, the translation prints brothers, because there the masculine is doing real work.
There is a register gain as well. Family vocabulary dropped in the middle of an argument about contracts or food or lawsuits does something to the argument, and "brothers and sisters" carries that where brethren, which now reads as a title, does not.
Read it in place
I'm urging you, brothers and sisters, by the authority of our Lord Jesus the One: all of you need to be on the same page. Stop letting divisions tear you apart. Instead, be completely united—same mind, same purpose.
1 Corinthians 1:10
When I came to you, brothers and sisters, I didn't come with impressive rhetoric or philosophical arguments to tell you about what God has made known.
1 Corinthians 2:1
Brothers and sisters, each person should remain with God in whatever situation they were in when they were called.
1 Corinthians 7:24
I don't want you to miss this, brothers and sisters — our ancestors were all under the cloud, and they all passed through the sea.
1 Corinthians 10:1
Questions people ask
What does brethren mean in the Bible?
Brethren is an old plural of brother used by Tyndale and the King James Version to translate the Greek adelphoi. In the letters it addresses everyone in a gathering, women included, as members of one family. It is not a term for a men's group or for clergy, though the English word now sounds like both.
Does brethren mean brothers and sisters?
In most New Testament passages, yes. Greek used the masculine plural adelphoi for any mixed group, so a letter addressed to the adelphoi of a city was addressed to its women as well as its men. Romans 16 names eight women in a single chapter of greetings and refers to a ninth, the mother of Rufus, which shows who was in the group being addressed.
What does brethren mean in the KJV?
The King James translators used brethren for adelphoi in Greek and for ach in Hebrew, covering blood brothers, fellow Israelites, and members of a gathering. In 1611 the word was already the marked plural, used for members of a company or order rather than for siblings. Modern English lost that distinction and kept only the antique sound.
What does brethren mean in Greek?
The Greek is adelphoi, the plural of adelphos, which is formed from the word for womb and means someone born from the same one. Greek applied the masculine plural to mixed groups by default. It was also standard vocabulary in trade guilds and associations for fellow members who were not related at all.