“As to the consequences of this predestination, Paul adds: ‘In order that he would be the eldest of many brothers.’ Just as God wanted to communicate his nature to others in making them participate in likeness to his goodness, so that he is not only Good, but the author of goods, so too the Son of God wanted to communicate the conformity of his filiation, so that he would not be an only child but the Firstborn of the sons. Thus, he who is unique in his eternal generation, according to John (1:18) ‘The only Son who is in the bosom of the Father,’ becomes through the communication of grace ‘the Firstborn of many sons’: ‘He who is the Firstborn among the dead, the Prince of the kings of the earth’ (Apoc. 1:5).
We are thus the brothers of Christ, as much by the fact that he has communicated to us the likeness of his filiation, as we say here, as by the fact that he assumed similarity to our nature, according to the epistle to the Hebrews (2:17): ‘He had to become like his brothers in all things.'” -St. Thomas, In ad Rom. VIII, lect. 6, nn. 703-6, as quoted in Saint Thomas Aquinas Vol. 2 Spiritual Master by Jean-Pierre Torrell, O.P. trans. Robert Royal, pg. 144. (the citation from Romans in Torrell comes after a longer quote passage, so presumably the nn. range given above is too expansive).