We are both born from the same God and the same Word. So we are brothers in that sense.
As to Primacy, the first born does not get the primacy in most cases in the bible. If we follow the parable of the prodigal son, the son who remained home and followed the law did not lose his birthright, he was just reminded that the other son who had just found his way home was just as important.
"But he answered his father, “Behold, these many years I have served you, and I never disobeyed a commandment of yours, but you never gave me a goat, that I might celebrate with my friends. But when this, your son, came, who has devoured your living with prostitutes, you killed the fattened calf for him.”
The fathers response in the parable is particularly telling:
“But it was appropriate to celebrate and be glad, for this, your brother, was dead, and is alive again. He was lost, and is found.”
The Gentiles were lost to the family of God. God didn’t say he rejected the first brother and chose the second. He said both should celebrate.
Rightly though, that doesn’t necessarily mean that the father is talking about the Gentiles and Jews, but as to the state of man accepting God in his heart.
The other thing to consider is that the Apostles were set up as the new 12 members of the covenant. So in a way, spiritual Israel is now not Jewish, but encompasses all mankind. So we are the same family, our Messiah is their Messiah, but we all have to accept him.