We are all children of God, but the Jews are special kind of child. Their culture, their entire existance, is based on a Covenant with God. The are the first Covenential Children of God, and His Chosen People. God made all people, but God made the Hebrews in a much more detailed and explicit way than He made the Gentiles. The entirety of the Hebrew People, culture AND blood, was brought forth in a unique and special way by God.
From that Covenant and People, God gave the world, all humans, the Messiah. The Messiah, even according to modern Jewish belief, is to bring all Gentiles into the Covenant with God that the Hebrews have. Therefore, the Hebrew Covenant was not destroyed, but rather grew into the Christian Covenant, a universal and Salvific Covenant. Modern Jews are therefore refered to as our “elder brothers” for the sake of their patriarchs, who originated the Covenant between God and Man long before Gentiles even recognized the One God.
The Hebrew Covenant still exists, though the Church recognizes that the modern Jews do not follow the “completed” Covenant (which I would think most Jews would agree with insofar as they believe that God hasn’t “finished” things by means of the Messiah, even if they disagree that Christianity represents the completion). The fact that they do not follow the completed Covenant DOES NOT MEAN that they do not follow the Covenant, nor that they necessarily willfully reject the completion of it (invincible ignorance, hardening of their hearts). We must honor them as the only other “authentic” religion in the world, the only other true response to God. Whatever “defects” that Judaism contains are defects of absence, not defects of direction; their faith is absolutely perfect as far as they take it, and indeed many Christians can learn from it. No other faith besides the Catholic Church can be said to adhere so perfectly to the direct Word of God, though we believe, as do the Jews in a certain sense, that the Jews haven’t adopted the “final phase”. Christians believe the Final Phase began with Jesus, Jews believe it hasn’t begun yet.
There are many other aspects of them being the “elder brothers”. They were also the first people that Jesus spoke to, the first people God spoke to, and the first people to answer the call when He did (both with the Hebrew AND Christian Covenant). The Apostles were all Jews, and only Jews recognized Jesus at first, so they are most definately our elder brothers in the sense of the New Testament as well.