The term brother (Gk. adelphos) has a broader meaning than uterine brothers. It can mean a biological brother, but it can also mean an extended relative, or even a spiritual brother.
Take Genesis 13:8 for example. Here the word brother is being used to describe the relationship between Abraham and Lot, who were not biological brothers but uncle and nephew:
“So Abram said to Lot, “Let’s not have any quarreling between you and me, or between your herdsmen and mine, for we are brothers” (Gen 13:8, NIV; see also 14:12).
Because of the Bible’s broad semantic range of “brother,” we can rest assured that although St. Paul writes, “[Jesus] appeared to more than five hundred…brothers at the same time” (1 Cor. 15:6), we need not infer from this verse that Mary gave birth to more than 500 children!
Yes,
adelphos does have a broad semantic domain; however, there are extremely good reasons why we should not take the term to mean anything other than a biological brother when in reference to Jame's relation to Jesus. One very good reason is found in Galatians 1.19, where Paul refers to James as "
the brother
of the Lord."
It is very different to be a brother
in someone, as in the spiritual sense that we're all brothers "in the Lord," and being
"the brother
of the Lord." In Eph. 6.21 Paul refers to Tychicus as “the dear brother and faithful servant
in the Lord” and in 1 Cor. 6.5-6 Paul refers to any believer as a “brother” in Christ. But here in Galatians 1.19 Paul uses the genitive case
tou kyriou which would signify the corresponding English preposition is "of" the Lord rather than "in" the Lord.
Another point in favor of this being a biological title is that in this context Paul refers to only James as “the brother of the Lord,” and not Peter. Should Paul's use of
adelphos refer to James as a spiritual brother then we would expect both, Peter and James to have claim to such a title.