Then his mother and his brothers came to him but were unable to join him because of the crowd. He was told, "Your mother and your brothers are standing outside and they wish to see you." He said to them in reply, "My mother and my brothers are those who hear the word of God and act on it." (Lk 8:19-21)
Jesus was with a crowd. The crowd was so great that no one could come close to him. This prevented even his mother and brothers to come close to him. The crowd was formed of people who came to him to listen to his preaching. The crowd also came because he healed many. So the crowd was formed of people who belonged to the kingdom.
Kingdom was formed of people. These people were not made of relatives or people who belonged to a group. Kingdom had Jesus as its centre. All the people were turned towards Jesus. They all listened to him. He was communicating something to them. His teachings, his doings and he himself were the main reasons for their coming together.
Jesus announces these people to be his relatives. Kingdom-people are the relatives of Jesus. Jesus does not give importance to the natural relations rather kingdoms relations become the only relations for him. This does not in any way exclude the natural relations. The relations who of natural origin also could become the relations of the kingdom.