What is the main conflict in The Duchess of Malfi by John Webster?

The Duchess of Malfi, a widow, has two wicked brothers who selfishly do not wish for her to remarry. In their jealous conquest to control her, they plot revengeful and violent murders. The Duchess secretly marries her gallant steward and bears three children to him. Lies, deception, and death await many characters as the play unfolds.