Matthew 13:55
“Is not this the carpenter’s son? Is not his mother called Mary? And are not his brothers James and Joseph and Simon and Judas?”

Mark 6:3
“Is not this the carpenter, the son of Mary and brother of James and Joses and Judas and Simon”

Jesus is said to have had brothers.  The Greek word for  “brothers"  is  “adelphos”  and it is a compound word.   A  =  “same”  and  delph =  “womb.”   So, some Protestants argue that these  “brothers”   of Jesus must be from the same womb as Himself and thus children of Mary, His mother.  However, this is an etymological fallacy.  

The derivation of a word, its etymology, will give you the history of a word, but it does not necessarily give you the correct definition of that word.  For example, Plato (429 -347 BC) in his LAWS  XI  924 E for his model state:  “That brother  [adelphos]  who is born of the same father or of the same mother …”     Therefore, the word  “adelphos”  cannot necessarily mean from the same womb.   Plato also uses the same word to mean  “kindred, or relatives.”

In order to get past the prejudice of to how to interpret the word “adelphos”  which is used in reference to Jesus, it is beneficial to consider other Bible passages that also use that word, but in a different context.

The phrase “his mother's sister,”  in Jn 19.25  [below],  is a reference to the mother of the sons of Zebedee , a.k.a. Salome.  This is also evident when we compare  Matthew 27:56,  Mark 15:40,  and  John 19:25 [see quotes below.]  These passages show a consistency whereby each of these three Gospels lists all three of the women at the cross besides the Blessed Virgin Mary.  It would have been self evident that a loving Jewish and presumably widowed mother of an only child would have been at the cross so it was almost redundant to state that the Blessed Virgin Mary was there. 

Those other three women are Mary Magdalene, the “other Mary” who was the wife of Clopas and the mother of James and Joseph (or Joses), and Salome who was Zebedee's wife and who is described as Mary's sister.

However, we know that Mary and Salome were cousins (see family tree below)  so this is an example of  “adelphos”   (actually, the feminine version of the word in this case),  being used with a wider definition than just children from the same womb.

Early church writers record that Salome, Zebedee’s wife, was actually the cousin of the Blessed Virgin Mary. See quotation below chart. 

The fact that they are called “sisters” in the Bible verse above shows that the word  “sister”  had a less restricted meaning and included close relatives. (The Greek word used in the Bible is the feminine version of “adelphos.”)

“There were three sisters of Bethlehem, daughters of Matthan the priest, and Mary his wife, under the reign of Cleopatra and Sosipatrus, before the reign of Herod, the son of Antipater: the eldest was Mary, the second was Sobe, the youngest’s name was Anne. The eldest being married in Bethlehem, had for daughter Salome the midwife; Sobe the second likewise married in Bethlehem, and was the mother of Elizabeth; last of all the third married in Galilee, and brought forth Mary the mother of Christ.”
(Nicephorus Callistus [Historia ecclesiastica, 2.3.   PG 145.760, A translation from Charles Wheatly, A Rational Illustration of the Book of Common Prayer, 1794, p. 63.]  Also see Hippolytus of Thebes, [Migne's Patrologia Graeca PG 117,]  Andrew of Crete, Epiphanius Monachus, and Andronicus)

Here is another example.  We know that King Herod and Philip are called brothers (Greek: adelphos)  in Mark 6.17.  Yet, we know that they are not from the same womb [as the derivation of that word might suggest.]   Their father King Herod the Great had four wives and they were born of different mothers.

(See  DICTIONARY OF THE BIBLE  by John L. McKenzie,S.J., page 355 for a more detailed chart of Herod's family.)
 

James, The Brother of the Lord

 Galatians 1:19
“But I did not see any other of the apostles, only James the brother of the Lord.”

There are two Apostles named James. However, it is clear that neither one of the Apostles named James is the son of Joseph and Mary. They are stated to be the “son of Zebedee” and the “son of Alphaeus” in the lists of the Apostles.   See Mark 3:14-19, and Matthew 10:2-4.

 Mark 3:14-19
“He appointed twelve (whom he also named apostles) … Simon, …  James, son of Zebedee, and … James the son of Alphaeus …”

When we compare these texts with John 19:25  we can deduce that Clopas and Alphaeus must be the same person.   This can be easily explained, as Karl Keating points out, because the Aramaic name Alphaeus can be rendered in the Greek as either Alphaeus or as Clopas. Or Alphaeus could have taken a Greek name similar to his Jewish name the same way as Saul took the Greek name Paul.

In the second century church historian Hegesippus explained that Clopas was the brother of Joseph, Mary’s husband.  Therefore, by marriage, Clopas’s sons and Jesus were close relatives, and hence they were called brothers in close knit familial society of the Hebrews.

Therefore, the reference to the brothers and sisters (ADELPHOS) of Jesus does not mean that Mary had other children.

Luke Chapter One implies that Mary had taken a vow of virginity.

Read more at

Mary's   Perpetual   Virginity   Was   Implied

Also see the following objections answered: 
Mat. 1:25  Until,  
Luke 2:7  Firstborn

Links  To  Other  Sites  On  This  Subject

Catholic Answers   by Karl Keating

New Advent by  Kevin Knight

BROTHERS AND SISTERS OF JESUS  EWTN