Sebastian is Alonso's (the King of Naples) brother. Antonio says he can imagine the crown on Sebastian's head because of the events that have recently taken place. It is believed that Ferdinand, the direct heir to the throne of Naples, has drowned. The next heir is Claribel, Alonso's daughter. She has just become the Queen of Tunis, and Antonio believes it will be impossible for her to rule Naples because she is so far away. Considering how Antonio took over his brother's position as Duke of Milan, Sebastian and Antonio both believe that Sebastian is the next likely heir to the throne of Naples.