the reasoning why the story continues after the Duchess dies

Throughout the play Ferdinand makes numerous references to his incestuous desires towards his sister, the Duchess of Malfi. he calls her a "lusty widow" (I, ii) which is rather strange to call your sister that. This is the early signs that Ferdinand has an incestuous love for the Duchess. When she dies Ferdinand plots to kill Antonio after she is dead. Not while she is alive, but after she dies. This is the reasoning why the story continues after the title character dies. It displays Ferdinand's incestuous love towards his sister. It's important to understand that he "loves" his sister so much that he wouldn't kill her lover while she is alive, but he'd kill Antonio out of jealousy when she is dead. It's almost as if he is respecting her, even though it's twisted and disgusting.