The Duchess of Malfi Morality and Ethics Quotes

How we cite our quotes: (Act.Scene.Line)

Quote #10

My sister! Oh my sister, there's the cause on't!

"Whether we fall by ambition, blood, or lust,

Like diamonds we are cut with our own dust." (5.5.70-72)

Ferdinand's final words are, perhaps not surprisingly, about his sister. When he first learned that the Duchess had given birth, he said that it was as if "some sin in us heaven doth revenge / By her" (go look at Quote #2 in Family for the full picture). While the latter part of these lines can easily translate to "we're all brought down by our own deeds," what do you make of the fact that Ferdinand's "own dust" is, to him, equivalent to "my sister"? Do you think that "the cause" that he mentions refers to what the Duchess herself did, or what Ferdinand did to her?