Albert Barnes on Romans 16:13

Chosen in the Lord. Elect in the Lord; that is, a chosen follower of Christ.

And his mother and mine. "His mother in a literal sense, and mine in a figurative one." An instance of the delicacy and tenderness of Paul; of his love for this disciple and his mother, as if he were of the same family, Religion binds the hearts of all who embrace it tenderly together. It makes them feel that they are one great family, united by tender ties, and joined by peculiar attachments. See what the Lord Jesus declared in Mt 12:47-50, and his tender address to John when he was on the cross, Jn 19:28,27.

(u) "chosen in the Lord" Eph 1:4, 2Jn 1:1