en_tn/gal/04/28.md

brothers

See how you translated this in Galatians 1:2.

children of promise

Possible meanings are that the Galatians have become God's children 1) by believing God's promise or 2) because God worked miracles to fulfill his promises to Abraham, first by giving Abraham a son and then by making the Galatians children of Abraham and thus sons of God.