True, Paul does address fellow believers as brothers but he also uses specific words from time to time.
As I've said before...context is your key to understanding the Word.
Then three years later I went up to Jerusalem to become acquainted with Cephas, and stayed with him for fifteen days. But I did not see any other of the apostles except James, the Lord's brother. Galatians 1:18-19 NASB
The Greek would render it this way if we put it into English, "James, brother of the Lord." In the Greek of the Lord is in the genitive case. Generally, the genitive case indicates one of possession. In this particular instance it is indicating a family relationship due to the structure of the Greek.
James is being identified as the brother of the Lord.
However, the reality is that the dispute about PMV cannot be determined by the use of adelphos alone, due to the lack of precision, while the context in which adelphos is used plays a role in this debate, and more so does the principal that Holy Spirit characteristically mentions exceptions to the norm, as exampled in post 274 by the grace of God.
Celibate marriage is contrary to its description, (Gn. 2:24; Mt. 19:4,5) and is unseen in Scripture, except that most likely David and Bathsheba were married, but the Spirit is careful to note that David did not sexually know her, (1Ki. 1:4) and which is consistent with the Holy Spirit characteristically recording notable deviations to the norm. Therefore the burden of proof is upon the Catholic to establish by Scripture perpetual Marian virginity, as per his tradition — but which he cannot and does not do, as the veracity of RC doctrine does not rest of upon the weight of Scriptural support, but upon the novel and premise of ensured perpetual magisterial infallibility, which is unseen and unnecessary in Scripture.
As regards adelphos, while this word can also often mean brethren in the larger sense, when adelphos is used with a parent (not necessarily named), or when one is named as a brother of someone then that it is less likely to be used in the wider sense, (avoiding duplicates: Mt. 1:2,11; 4:18,21; 10:2; 14:3; 17:1; Mk. 5:37; 15:40; Jn. 11:2;
The often mentioning of “His mother, and his brethren” together, along with the naming of 4 brethren strongly suggests immediate family, rather than extended, and thus some resort to another scenario, that these brethren were because Joseph was a widower with sons from a previous marriage*. This is more reasonable as far as acknowledging “His mother, and his brethren” as referring to immediate family, but there is no reason to resort to this explanation except to disallow Scripture from contradicting a tradition of men, contrary to the most reasonable meaning, that of a normal consummated marriage, resulting in children.
Matthew 13:55-57: "Is not this the carpenter's son? is not his mother called Mary? and his brethren, James, and Joses, and Simon, and Judas? And his sisters, are they not all with us? Whence then hath this man all these things? And they were offended in him. But Jesus said unto them, A prophet is not without honour, save in his own country, and in his own house. " Meanwhile unlike here, Luke 7:12 makes note of the case when a man was “the only son of his mother.”
"But other of the apostles saw I none, save James the Lord's brother. " (Galatians 1:19)
"And the multitude sat about him, and they said unto him, Behold, thy mother and thy brethren without seek for thee. " (Mark 3:32)
"After this he went down to Capernaum, he, and his mother, and his brethren, and his disciples: and they continued there not many days. " (John 2:12)
"These all continued with one accord in prayer and supplication, with the women, and Mary the mother of Jesus, and with his brethren. " (Acts 1:14)
Rather than the weight of Scripture warranting PMV, instead it flows from doctrine close to that of demons (1Tim. 4:1-3) which certain so-called "church fathers" held, as shown in post 247. (perpetual Marian virginity)
*The International Standard Bible Encyclopedia states, In the Apocryphal Gospels, the attempt is made to supply what the canonical Gospels omit. They report that Joseph was over eighty years of age at his second marriage, and the names of both sons and daughters by his first marriage are given. As Lightfoot (commentary on Galatians) has remarked, “they are pure fabrications.” Theophylact even advanced theory that they were the children of Joseph by a levirate marriage, with the widow of his brother, Clopas. Others regard them as the nephews of Joseph whom, after the death of his brother Clopas, he had taken into his own home, and who Thus became members of his family, and were accounted as though they were the children of Joseph and Mary. According to this view, Mary excepted, the whole family at Nazareth were no blood relatives of Jesus. It is a Docetic conception in the interest of the dogma of the perpetual virginity of Mary. All its details, even that of the advanced age and decrepitude of Joseph, start from that premise.