Antonio often dreams of disaster, but he also dreams of becoming a priest. In his dreams, Antonio has powers that he does not have in real life. He is sought after and seems to either have or be the answer that others are looking for.
This is true especially of the dreams Antonio has of his three brothers.
In these dreams Antonio seems to be realizing that his brothers are "lost" and will not return home until they have found something of their own in the world. This also suggests Antonio's increased responsibility to his parents as, without his brothers, Antonio is the last male child left to the family.
Other dreams imagine disaster, death, and a world in chaos. Antonio is distressed, clearly, by the events that take place regarding Ultima and Tenorio. Tenorio's threats place a stress on Antonio which adds to the series of emotional and spiritual difficulties he is already having regarding his religious and familial identity.
His dreams serve to express these stresses and reinforce the motif of magic that runs through the text.