rather than the Greek of the New Testament.
The Greek words adelphos and adelphe were not used there.
Keep in mind also that because it is legitimate to speak of a child as one's "firstborn" does not mean that they need to
always be called that. Nor is it necessary to
always refer to an only child as "my only child".
I would also note that the early Fathers of the Church all considered Jesus to be an only Child. It has been a Tradition of the Church from the beginning.
For instance St. Jerome wrote a letter "Against Helvidius" on The Perpetual Virginity of Blessed Mary around 383 AD
http://www.clerus.org/clerus/dati/2001-02/17-999999/virginity.html
See also Br. Anthony O****o, M.D.'s treatise on The Perpetual Virginity of Mary
http://www.cin.org/users/james/files/talmud.htm