Isa 11:11-12 has already happened starting with Judah's return to the land from Babylon and continuing with the outcasts (lost sheep) of Israel's return for centuries afterward culminating at Pentecost.
Prophesied
And it shall come to pass in that day, that the Lord shall set his hand again the second time to recover the remnant of his people, which shall be left, from Assyria, and from Egypt, and from Pathros, and from Cush, and from Elam, and from Shinar, and from Hamath, and from the islands of the sea. And he shall set up an ensign for the nations, and shall assemble the outcasts of Israel, and gather together the dispersed of Judah from the four corners of the earth. Isaiah 11:11-12Fulfilled
And there were dwelling at Jerusalem Jews, devout men, out of every nation under heaven. Acts 2:5And how hear we every man in our own tongue, wherein we were born? Parthians, and Medes, and Elamites, and the dwellers in Mesopotamia, and in Judaea, and Cappadocia, in Pontus, and Asia, Phrygia, and Pamphylia, in Egypt, and in the parts of Libya about Cyrene, and strangers of Rome, Jews and proselytes, Cretes and Arabians, we do hear them speak in our tongues the wonderful works of God. Acts 2:8-11
Here is the verse before it for more context:
Isaiah 11:10 And in that day there shall be a root of Jesse, which shall stand for an ensign of the people; to it shall the Gentiles seek: and his rest shall be glorious.
These 3000 Jewish men were returned to Jerusalem from many lands where their families had lived since the captivity.
They no doubt would have brought the gospel also to their families.
All these Jewish Christians would continue to marry among Christians and not identify as Jews.
So how do we pretend to know the true identity of Paul's kinsmen in Romans 11?