Malachi Martin, who ain't no heretic, is in hot water with the Vatican again over his studies of disputes in the very early church over all the plum jobs going to Christ's various Uncles, Aunts, and the cousins galore. The Church at Ephesus was home to the Virgin Mary (and Diana of Ephesus, items of whose cult slipped easily into Christian practice, and from which many converts were undoubtedly made). She was from a large family, Joe was from a large family, and these relatives of which there was no shortage, were of course among the very first converts ... not to mention apostles and disciples. Since these were all collateral relatives, this helps wily old Malachi Martin evade the old sibling question of "the brothers and sisters of Christ," keeping him off the Pope's major radar.
Malachi Martin has dug up some interesting stuff which purports to show that several generations later, the Greek converts apparently weren't too happy about Greek pastorships and bishoprics going to these folks and complained about it to the Pope in Rome, with whom they were in correspondence over the course of that first two centuries or so. Malachi says the correspondence still exists.
Fascinating.
What you're saying is quite true. To claim position in the Church as Jesus' close kin, would have been accepted, even expected in those days when family/tribal systems of affiliation were the norm and pedigree/genealogy were of such intense interest. This makes it all the more striking that all the claims are collateral: people might put forward an uncle/aunt/stepbrother connection, cousins by the dozens, but nobody ever ventured to suggest they were Christ's son or grandson.
If it were even remotely credible that Jesus might have had a son, there would eventually have been a dynastic dispute. But it never happened.
Malachi Martin, by the way, is being accurate, not just "Papally Correct", when he speaks of half-brothers, step-brothers or cousins, rather than full blood siblings, of Jesus. Although in Greek there are separate words for "cousin" and "nephew," the translators of the Old Testament used the Greek word, "adelphos," i.e. "brother," even when it was clear that no blood brother is meant. This happens some 20 times in the Greek Septuagint, which was the version familiar to all of the Evangeliss as well as Paul: Abraham refers to his nephew, Lot, as his brother; Laban calls his nephew, Jacob, his brother; in Leviticus cousins are called brothers; etc., etc. The Greek translators, certainly familiar with the Hebrew usage, still has no problem using "adelphoi" for everybody.
The individual brothers named in the Gospels, are specified as the sons either of Clopas or of "the other Mary," or (as some maintain) of both Clopas and "the other Mary."