Whether it is an insertion isn't even the point - since Luke 2:7 also includes the phrase.
It isn't an Englishism when I looked it up in the lexicon. What is a prototype? It is the first in a line of items.
Prototokos came from two Greek Roots.
Protos- which has these meanings:
1) first in time or place
a) in any succession of things or persons
2) first in rank
a) influence, honour
b) chief
c) principal
3) first, at the first
And Tikto meaning:
1) to bring forth, bear, produce (fruit from the seed)
a) of a woman giving birth
b) of the earth bringing forth its fruits
c) metaph. to bear, bring forth
So Mary gave birth to either her:first in time or place,
in any succession of things or persons, highest ranked, her chief, principal, or most influential Son. This is Jesus in relation to Mary.
I do not argue that the "prototokos" is an invalid or late insertion. Apparently Kosta was under such impression.
Nor do I argue that it means anything but "first born".
The point is that it applies even to a single child because it has a dynastic meaning, placinf Jesus in the line of King David.
You cannot draw conclusions about existence of other siblings form the fact the Christ is validly described as the "prototokos". To do so is to violate the cultural, historical, and unltimatley theological context in which the word is used by St. Matthew.