A note (oft repeated in Mariology) on the secular and LXX usage of the term adelphoi:
1. Hellenistic (examples):
Extant literature from the period refers to Athene and Apollo as "adelphoi". They did not share a mother ( Athene was born fully grown from the head of her father, Zeus).
Where precision is required, Plato - in his "Laws" - narrows the term "adelphoi" through the use of appropriate descriptives when the term is used in reference to siblings.
Extant literature uses the term "adelphoi" broadly; it can mean: household mate, neighbor, sibling, half sibling, fellow countryman, of like disposition, etc.
2. LXX
The term "adelphoi" is used to describe the relationship between Lot and Abraham.
3. NT
In the Gospel of John, the author attests that Mary had an "adelphi" named Mary.
Finally, the only fully certain way of a statement establishing 'siblingship' is through a comparative of those persons to whom the descriptive "son of, daughter of" or "mother of, father of" is made. One would need then to find all persons for whom Mary is stated to be "mother of", or all persons who are called "daughter/son of Mary" and then correspond the names (and be certain that these names refer to the same person, not different persons of the same name).
When one reads in translation from a different historical era and culture, the default understanding of one's own experience may be misapplied thus altering the sense or meaning of the text that is read. IE, what seems to be plain in English is not plainly stated in the original text which is the 'product' of a different historical, linguistic and cultural context.