These were Jesus' younger brothers—the other sons of Mary and Joseph who were born after Jesus. Since the Father of Jesus was God, and their father was Joseph, they were technically his half-brothers. This detail is not normally translated.
This metaphor expresses that the people who were coming to listen to Jesus were as important to him as his own family was. AT: "Those who hear the word of God and obey it are like a mother and brothers to me" or "Those who hear the message from God and obey it are as important to me as my mother and brothers" (See: [[rc://en/ta/man/translate/figs-metaphor]])