The twelve tribes were never lost - they were carried to Assyria, then later Babylon - but the OT says many came back to Israel. Then the Jews were scattered in the second century by the Romans after the Bar Cochba (I might be off on my spelling) rebellion.

We are spiritually heirs of Abraham - we inherit his spiritual blessings and promises, not his physical ones. The literal Jews will inherit them when the Lord turns back to them.

This verse:

James 1:1 James, a servant of God and of the Lord Jesus Christ, to the twelve tribes which are scattered abroad, greeting.

is referring to this scattering of believers during Saul's persecution of the early church:

Acts 8:1 And Saul was consenting unto his death. And at that time there was a great persecution against the church which was at Jerusalem; and they were all scattered abroad throughout the regions of Judaea and Samaria, except the apostles.

Acts 8:4 Therefore they that were scattered abroad went every where preaching the word.

The church up to that point in the book of Acts was mostly made up of Jews - safe to say, from the twelve tribes, which were then scattered abroad.