According to Matt. 13:55, the Lord had four brothers (i.e. half-brothers, as we say), James, Joses, Simon and Judas. He had at least three sisters also - "and His sisters, are they not all with us?" Had there been but two, the word all would have been both.
The Lord is called mary's "firstborn" (Matt. 1:25 and Luke 2:7), and the natural inference is that Mary had other children. The word prototokos is used only in these two passages and in Rom. 8:29; Col. 1:15, 18; Heb. 1:6; 11:28; 12:23 (pl.); Rev. 1:5, so that the meaning is easily ascertained. Had He been her only son, the word would have been monogenes, which occurs in Luke 7:12; 8:42; 9:38, of human parentage; and of the Lord, as the only begotten of the Father, in John 1:14, 18; 3:16, 18; 1John 4;9. In Heb. 11:17 it is used of Isaac, Abraham's only son according to the promise.
In Psalm 69, a Psalm with many predictive allusions to the Lord's earthly
life (see Note on Title), verse 8 reads, "I am become a stranger unto my
brethren, and an alien unto my mother's children". The Gospel
history records His brethren in association with His mother. After
the miracle at Cana, which they probably witnessed, we are told that "He
went down to Capernaum, He, and His mother, and His brethren, and His disciples"
The natural meaning of the term "His brethren", in the Scripture record,
would never have been challenged, but for the desire, when corruption crept
into the churches (Acts 20:29, 30), of raising Mary from the position of
"handmaid of the Lord" (Luke 1:38) to the exalted one of Theotokos,
mother of God, whence it was an easy step to investing her with divine
honors, as being herself a goddess. And thus the way was cleared
for identifying her with the great goddess of Paganism, who is the mother
of a divine son, and who is yet a virgin, a deity best known by the appellation
she bore in Egypt, Isis, the mother of Horus.
With reference to Jerome's "cousin" theory, it may be stated that the
word "brother" is used in Scripture, (1) in the sense of blood-relationship,
as children of the same parent or parents; (2) in the wider sense of descent
from a common ancestor, e.g. Acts 7:23, 25, where Abraham is the forefather;
(3) in a still wider signification of fellow-man (Matt. 7:3-5; 18:15);
(4) to express spiritual relationship (Matt. 23:8; 28:10; Acts
9:17; Rom. 8:29; Heb. 2:11). In the passages where His
brethren are referred to, viz. Matt. 12:46, 47; 13:55; Mark
3:31; Luke 8:19; John 7:3, 5, 10; Acts 1:14; 1Cor
9:5; Gal. 1:19, only the first meaning can apply.