Jesus does not have a brother in the catholic tradition, his mother Mary is a perpetual virgin, which can hardly be compatible with having several children.
Yet in his epistle to the Galatians (1, 19), Paul calls James "the brother of the Lord". In the first letter to the Corinthians (9: 4-5), he mentions other brothers of the Lord who have the right to take their wives during their apostolic mission. It is clear in Paul's formulation that compares his rights with "other apostles and the Lord’s brothers, and the apostle Peter" that he does not attribute the qualifier of brother of the Lord only to a few specific individuals. It does not stand as synonym of disciple.
From this, several historians hold the position that either Joseph or Mary had children together or from previous relationships for Joseph like P-A. Bernheim1 , or F. Blanchetière2 , who point out that, Paul never qualifies Peter or John as brother of the Lord, or Fr John P. Meier who denies3 the theory of cousins that never appears in the Greek version of the Old Testament in which the term adelphos marks exclusively the fraternal bond of blood or right.
However the Catholic church and many Catholic exegetes believe that Mary didn't had other children, according to the Catechism of the Catholic Church, following the traditional reading based on the later belief in the perpetual virginity of Mary, following Jerome 4, the first Father of the Church to argue against the siblings theory for the benefit of the cousins one at the end of the fourth century. They also recuse the half-brothers and sisters theory presented in an apocryphal gospel from James, around 180, born of a first marriage of Joseph from previous relationships.
Why these "Brothers of the Lord" have been embarrassing for the Catholic Church which make them gradually forgotten and lose their quality of brothers? Why at the same time as the cult of chastity develops, whose mother of Jesus becomes the symbol and which will find its apogee in the doctrine of its perpetual virginity?
Note : Other like Assyrian and part of protestants hold this doctrine as well but I narrow my question to the Catholic Church as far as the different schisms had not yet been pronounced when the doctrine was created.