When Paul uses the Greek 'adelphoi' in his letters e.g "What then shall we say, brothers and sisters?" (1 Corinthians 14:26) is he meaning brothers and sisters (as the NIV translates) or just men?
Each one of the adelphoi Paul is addressing has, according to Paul, a hymn, a lesson, a revelation, a tongue, or an interpretation. In the same Epistle, Paul will also state that women must remain silent in the churches (14:34). I think we must therefore conclude that Paul is addressing men only, and not men and women.
1 Corinthians 14:26 (RSV)
What then, brethren? When you come together, each one has a hymn, a lesson, a revelation, a tongue, or an interpretation. Let all things be done for edification.
1 Corinthians 14:34 (RSV)
The women should keep silence in the churches. For they are not permitted to speak, but should be subordinate, as even the law says.
First Corinthians was written as a letter, so whenever Paul addresses his readers, he is addressing those to whom it was sent, which in turn can perhaps be identified by the opening passage:
1 Corinthians 1:1-2: Paul, called to be an apostle of Jesus Christ through the will of God, and Sosthenes our brother, Unto the church of God which is at Corinth, to them that are sanctified in Christ Jesus, called to be saints, with all that in every place call upon the name of Jesus Christ our Lord, both theirs and ours:
This seems all-inclusive, given that we can tell from verse 1:11 that Paul recognised the presence and role of women in his church:
1 Corinthians 1:11: For it hath been declared unto me of you, my brethren, by them which are of the house of Chloe, that there are contentions among you.
To some extent, 1 Corinthians 14:34-35 may appear to be contrary, at least suggesting a quite secondary role for women:
1 Corinthians 14:34-35: Let your women keep silence in the churches: for it is not permitted unto them to speak; but they are commanded to be under obedience, as also saith the law. And if they will learn any thing, let them ask their husbands at home: for it is a shame for women to speak in the church.
However, as summarised here and here, many scholars regard 1 Corinthians 14:34-35 to be an obvious interpolation. In Forged, page 245, Bart D. Ehrman points out that 1 Corinthians 14:34-35 interrupts an argument on prophecy:
1 Corinthians 14:29-33: Let the prophets speak two or three, and let the other judge. If any thing be revealed to another that sitteth by, let the first hold his peace. For ye may all prophesy one by one, that all may learn, and all may be comforted. And the spirits of the prophets are subject to the prophets. 33 For God is not the author of confusion, but of peace, as in all churches of the saints ...
1 Corinthians 14:36-37: What? came the word of God out from you? or came it unto you only? If any man think himself to be a prophet, or spiritual, let him acknowledge that the things that I write unto you are the commandments of the Lord.
Further to this, Ehrman says:
It is hard to believe that Paul would tell women that they could not speak in church here in 1 Corinthians 14, when just three chapters earlier he indicated that they could do so. In 1 Corinthians 11 Paul urges women who pray and prophesy in church to do so only with veils on their heads. If they were allowed to speak in chapter 11, how could they be told not to speak in chapter 14?
Given the doubts as to the authenticity of 1 Corinthians 14:34-35, these verses can not be evidence that Paul was not writing to women as well as to men. In the overall context of the letter, it appears that in 14:26 and elsewhere where grammar and sense allow, men and women were being addressed.