"Is not this the carpenter, the son of Mary, and brother of James and Joses and Judas and Simon? Are not His sisters here with us?" And they took offense at Him."
There have been arguments upon arguments about what adelphos actually means. People who believe that Mary had other children say that in the context that it is used, it means brother and only brother. While Catholics contend that it may mean much more than simply brother and can extend to immediate family including cousins and other blood relatives.
According to Strongs, the greek word for brother(s) is adelphos (#80) and it means:
1. a brother, whether born of the same two parents or only of the same father or mother
2. having the same national ancestor, belonging to the same people, or countryman
3. any fellow or man
4. a fellow believer, united to another by the bond of affection
5. an associate in employment or office
6.brethren in Christ
a. his brothers by blood
b. all men
d. Christians, as those who are exalted to the same heavenly place
So we can see that adelphos does indeed have a meaning which is wide-ranging.
However, the non-Catholic would contend that since Greek has a seperate word for cousin, that it would have made sense that if the brothers of Jesus were actually cousins (anepsios #431), that that word would have been used instead of adelphos.
I contend however that given the usage of Greek at the time, that this would not have been the case. To prove this, we can simply look at other examples of Greek writing from a similar time period and see the word usages. Our best source would be to look at the Greek Septuagint, Old Testament and find examples of where adelphos/anepsios are used.
One clear example presents itself in Genesis 13:8 in which the English (NAS) version states:
So Abram said to Lot, "Please let there be no strife between you and me, nor between my herdsmen and your herdsmen, for we are brothers.
One thing should be noted at this point however. Abram and Lot were not brothers in the sense that they shared the same mother/father. Lot was Abram's brother's son. Which means Abram was actually Lot's uncle. But in the verse, Abram calls Lot his brother.
Anyways, allow us to look at the Greek version (transliterated by myself using the Unbound Bible website):
Eipe de o Abram pros ton Lot, As mh hnai, parakalo eris metaxu emou kai sou kai metaxu twn poimenwn mou kai twn poimenwn souc dioti adelphoi eimeqa hmeisc.
So, even the LXX uses the word adelphos above and beyond it simply meaning a strict blood-brother. Given the estimated dates for the writing of the LXX, and the frequent use and knowledge of this writing by the New Testament authors, we can easily conclude that the NT authors used a similar style and word-usage, meaning that the word adelphos as meaning strictly brother is faulty.