Ferdinand says this is act 4, scene 2. Bosola, on Ferdinand's orders, has the duchess killed. He then takes Ferdinand in to see his sister's corpse. Ferdinand has been unmoved by the corpses of the duchess's murdered children, saying only that the "death / Of young wolves is never to be pitied."
However, when Bosola directs Ferdinand to look at the duchess, the duke has a different reaction. Bosola says "Other sins only speak; murder shrieks out." Ferdinand responds by saying, "Cover her face. Mine eyes dazzle."
Ferdinand is emotionally moved by his sister's death. He says she died young and comments that they are twins. He suggests she was born first, saying that were he to die now, a few minutes after her death, they would have lived exactly the same amount of time.
Ferdinand shows some of the effects of his madness as he dissociates himself from responsibility for ordering the murders and blames Bosola for the crime, asking,
Why didst not thou pity her? What an excellent
Honest man might’st thou have been
If thou hadst born her to some sanctuary.
Ferdinand reveals here that while he wants her lands and titles, he also desires to live in a fantasyland where he doesn't have to be responsible for killing her to get them. He wants to blame an underling for not disobeying his orders instead.
Ferdinand shows some vestiges of human feeling towards the sister he grew up with. When he says "cover her face," it is clear that her death disturbs him in a way her children's death didn't. When he says his eyes "dazzle," he uses a double entendre: "dazzle" means that he is blinded both by her youth and beauty and the unshed tears that fill his eyes at her demise.