The house of Israel and Judah are The "sons of Abraham".
The Abrahamic Covenant has always included Gentiles.
"As for Me, behold, My covenant is with you, and you shall be a father of many nations. Genesis 17:4
The Law was "added" to the "Abrahamic Covenant" until...
What purpose then does the law serve? It was added because of transgressions, till the Seed should come to whom the promise was made... Galatians 3:19
By saying added, The Holy Spirit is indicating it was part of something greater.
By saying till, The Holy Spirit is indicating it was temporary.
The New Covenant included Gentiles, as Isaiah says -
1 "Behold! My Servant whom I uphold, My Elect One in whom My soul delights! I have put My Spirit upon Him; He will bring forth justice to the Gentiles. 2 He will not cry out, nor raise His voice, Nor cause His voice to be heard in the street. 3 A bruised reed He will not break, And smoking flax He will not quench; He will bring forth justice for truth. 4 He will not fail nor be discouraged, Till He has established justice in the earth; And the coastlands shall wait for His law." 5 Thus says God the Lord, Who created the heavens and stretched them out, Who spread forth the earth and that which comes from it, Who gives breath to the people on it, And spirit to those who walk on it: 6 "I, the Lord, have called You in righteousness, And will hold Your hand; I will keep You and give You as a covenant to the people, As a light to the Gentiles. Isaiah 42:1-6
JLB
The New Covenant is not made with the sons of Abraham, it is made with the house of Israel and the house of Judah.
Your reference to Isa 42 is taking some liberties...
Isa 42:1 Behold My Servant [Messiah], Whom I uphold; Mine elect, in Whom I Myself am well pleased; I have bestowed My Spirit upon Him: He shall bring forth judgment to the Gentiles.
Isa 42:2 He shall not make outcry, nor lift up, nor cause His voice to be heard in the street.
Isa 42:3 A bruised reed shall He not break, and the wick made of flax, that is burning dim shall He trim it and make it burn brightly: He shall bring forth judgment in accordance with truth.
Isa 42:4 He shall not go out as a lamp nor break down, till He have set judgment upon the earth: and the maritime countries [the Gentiles] shall wait for His law.
Isa 42:5 Thus saith God the LORD, He That created the heavens, They that stretched them out; He that spread forth the earth, and that which cometh out of it; He That giveth breath unto the people upon it, and spirit to them that walk therein:
Isa 42:6 I the LORD have called Thee in righteousness, and will hold Thine hand, and will keep Thee, and give Thee for a covenant of the people, for a light of the Gentiles;
There is not one word here about the Covenant being made with the gentiles, what is said is that Christ will bring forth judgment to the Gentiles (verse 1). He will extend His law to the Gentiles (verse 4). He will give a covenant to a particular people who will be for a light to the gentiles. The Covenant is made with Israel and Judah...
Jer 31:31 Behold, the days come, saith the LORD, that I will make a new covenant with the house of Israel, and with the house of Judah:
Jer 31:32 Not according to the covenant that I made with their fathers in the day that I took them by the hand to bring them out of the land of Egypt; which My covenant they brake, and I abhorred them, declareth the Lord:
Jer 31:33 But this shall be the covenant that I will make with the house of Israel; After those days, saith the LORD, I will put My law in their inward parts, and write it on their hearts; and will be their God, and they shall be My People.