1. A survey of LXX usage as well as the Hellenistic usage (which overlap),
there are easily over half a dozen meanibgs for the word, of which brother and cousin are two. This does not substantiate dogma, but weighs heavily against the narrow use for adelphos as sibling.
2. A careful reading of the passages in Luke describing the interaction between Mary and Gabriel establishes two options if one is to hold the narrow meaning 'sibling' for adelphos:
a. the Bible is in error
b. Mary lied
3. A knowledge of Hebraic culture contemporary with the era of the Gospel
yields information which sheds further light on the "adelphos passages":
a. children were rarely if ever named for their parents; the adelphos named Joses/Joseph is most likely not a child of Joseph, hence the term adelphos in its broad usage is the most accurate descriptive
b. unless Mary was an adulteress, Christ would have been "bearing false witness" against Mary by leaving her in the care of one who was not her son.
c. households were combined upon the death of the primary male provider; had Joseph died before the time of Christ's ministry, the term adelphos refers to household not immediate parentage.
4. the witness of the anti-Christian pagan, Celsus, indicates a teaching or knowledge that Mary was the biological mother of a single child (and that Joseph likely died some time before the ministry of Christ commenced).
5. Tradition held by three ancient Churches (in spite of later schism) attest that Mary was the mother of one child.
the weight of the evidence is decisevely against the claim that adelphos clearly means 'sibling'
the EO/OO/RC use "Tradition" as the canon for measuring and authentication - the evidence described above strongly supports the teaching per Mary as it stands.