There are examples in the Bible of firstborn having nothing to do with birth order but rather rank, or preeminance. Simply read Col 1 and you will see the meaning of 'firstborn':
NWT
15 He is the image of the invisible God, the firstborn of all creation; 16 because by means of him all [other] things were created in the heavens and upon the earth, the things visible and the things invisible, no matter whether they are thrones or lordships or governments or authorities. All [other] things have been created through him and for him. 17 Also, he is before all [other] things and by means of him all [other] things were made to exist, 18 and he is the head of the body, the congregation. He is the beginning, the firstborn from the dead, that he might become the one who is first in all things;
Verse 18 sum it up- first in all thing. He was not the first to rise from the dead. Also, the word "other" does not belong in verses 16-17. No other translation inserts the word other in those texts, the word does not appear in the Greek manuscripts. That is why the NWT brackets the word 'other'
NASB has a nice explanatory footnote on the use of the word 'firstborn' as opposed tot he Greek word "first-created"...you may find this informative.