Rudolph Pesch has championed the position that Jesus really had fleshly siblings. John Meier reports:
"Although his claims raised a fire storm of controversy among German Catholics, he has never been officially censured or condemned by Rome for his views" (Meier, A Marginal Jew, page 319). But here are the arguments posited by Meier:
(1) Meier argues that the Greek word ADELFOI ("brothers" or "brothers and sisters") combined with the "until" statement of Matthew 1:25 "creates the natural impression that Matthew understood 1:25a to mean that Joseph and Mary did have children after the birth of Jesus" (A Marginal Jew, page 322).
(2) In Matthew 13:55, Matthew appears to place Jesus’ brothers: "with his biological mother, not his legal father" (323).
(3) ADELFOS, the Hebrew word 'AH (ACH) and the Aramaic 'AHA are naturally translated as "brother." 'AH or ADELFOS could be used to mean "cousin," but only if the immediate literary context clarifies the relationship of the ADELFOI under consideration. Yet, Meier writes, "No such clarification is given in the NT texts concerning the brothers of Jesus. Rather, the regularity with which they are yoked with Jesus' mother gives the exact opposite impression" (325).
As an aside, BDAG Greek and English Lexicon observes that the Old Testament usage of 'AH does not establish the meaning, "cousin," for the Greek ADELFOS. This lexicon notes that "in rendering the Hebr. 'AH [ADELFOS] is used loosely in isolated cases" to designate male relatives "of various degrees" (BDAG 18). See ADELFOS, sense 1 in BDAG.
(4) The way that ADELFOS is employed by New Testament writers suggests that it refers to "brothers" rather than cousins. The word in fact describes "full brothers" in Mark 1:29-30 and throughout the New Testament. Meier thus wonders: “Why an exegete, operating purely on philological and historical grounds, should judge differently in Mark 6:3, where we hear that Jesus is the son of Mary and the brother (ADELFOS) of James, Joses, Jude, and Simon, is not clear” (Meier 327). He thus concludes: "In short, the 'cousin' approach of Jerome, like the 'stepbrother' approach of Epiphanius, simply lacks sufficient philological basis in the usage of the NT" (329).
(5) There appears to have been a tradition among the Church fathers of interpreting the brothers of Jesus as "real brothers" (331).
"Hence, from a purely philological and historical point of view, the most probable opinion is that the brothers and sisters of Jesus were his siblings. This interpretation of the NT texts was kept alive by at least some Church writers up until the late 4th century" (332).
Meier presents much more detail, but I have simply tried to summarize his view. The point I want to make is that Meier says history and philology suggests Jesus had real siblings. Faith may direct certain believers to hold onto the tradition espoused by Jerome in the 4th century, however. Can the two approaches be combined?