Another source for your perusal:
This item is available on the Apologetics Press website at:
apologeticspress.org/articles/2318
AP Content :: Scripturally Speaking
Did Jesus Have Fleshly Half-Brothers?
by Dave Miller, Ph.D.
The usual word in the Greek language for “brother” is adelphos. It possesses the same latitude of application that the English word possesses. Hence, it can refer to a person who shares the same religion (a spiritual brother). It can refer to a person who shares the same citizenship—a fellow countryman. It can refer to an intimate friend or neighbor. All of these uses are self-evident, and do not encroach upon the literal use of the term…
At least two assertions have been advanced by those who wish to discount the existence of Jesus’ brothers, and thereby defend the doctrine of Mary’s perpetual virginity. One attempt seeks to broaden the meaning of the Greek word for “brother” to mean “cousin.” According to this view, the “brothers” of Jesus were actually His cousins—the children of Mary’s sister. The assertion that “brother” has this enlarged meaning is made largely on the basis of the Greek translation of the Old Testament (the Septuagint). The Septuagint translators sometimes used the Greek word for brother (adelphos) in Old Testament passages in which a near relative or kinsman, who was not technically a physical brother, was under consideration. This claim is true. The Hebrew term for brother (‘ach) occasionally was used to refer to a more remote descendant from a common father who was not technically a brother (Gesenius, 1979, p. 27; Harris, et al., 1980, 1:31; Botterweck, 1974, 1:190). For example, Laban, Jacob’s uncle, was referred to as Jacob’s “brother” (Genesis 29:12,15). Likewise, Abram’s nephew Lot was said to be Abram’s “brother” (Genesis 14:14,16)…
Additionally, if the phrase “brothers and sisters” means “cousins” in Matthew 13:55-56 and Mark 6:3, then these “cousins” were the nephews and nieces of Mary. But why would the townspeople of Nazareth connect nephews and nieces of Mary with Joseph? Why would the townspeople mention nephews and nieces at all while omitting other extended family relatives? The setting assumes that the townspeople were alluding to the immediate family of Jesus. Barnes noted that to recognize these brothers and sisters as the sons and daughters of Joseph and Mary is the “fair interpretation,” and added, “the people in the neighbourhood [sic] thought so, and spoke of them as such” (1977, 1:150). As Matthews commented, “Joseph, Mary, and their children were recognized as a typical family of Nazareth, and when Jesus began his unusual career, they merely asked if He was not a member of this family mentioning their names. If these children were nephews and nieces of Mary, why are they always associated with her and not with their mother?” (1952, pp. 112-113, emp. added).
…oh, and by the way, why did Luke put this in the book (Luke 1:26-28) if Mary was a co-equal with Jesus? Wouldn’t she have already known that Gabriel was coming and what it was about?
26In the sixth month, God sent the angel Gabriel to Nazareth, a town in Galilee, 27to a virgin pledged to be married to a man named Joseph, a descendant of David. The virgin’s name was Mary. 28The angel went to her and said, “Greetings, you who are highly favored! The Lord is with you.”
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29 *Mary was greatly troubled at his words *and wondered what kind of greeting this might be. 30But the angel said to her, "Do not be afraid, Mary, you have found favor with God.