The Duke finds Mariana at her home, and Isabella arrives there soon after. Isabella relays that Angelo has given her two keys that she can use to access the garden of his house, where she has promised to meet him that night. Isabella has told Angelo that she will only be able to stay briefly, because she will be escorted by a servant who believes she has come to discuss Claudio.
Note the inversion of the relationship between Angelo and Isabella that’s taken place here. Angelo was the one who initially acted disingenuously in an effort to manipulate Isabella’s naiveté and ensure his public persona stayed pristine. Now, however, it’s Isabella who takes advantage of Angelo’s naïve faith that she will stay true to the bargain he has forced upon her.
The Duke introduces Mariana and Isabella and the two outline their plan off-stage. When the two return, Isabella tells the Duke that Mariana has agreed to the scheme. Isabella then gives Mariana one final instruction: when she leaves, she must whisper to Angelo the phrase “Remember now my brother.” Once Mariana assents, the Duke reassures her that sleeping with Angelo is, for her, no sin, because he was contracted to be her husband. The justice of their engagement outweighs the deceit of this ruse.
Mariana’s unique relationship to Angelo offers a fairly pat way to rationalize the extramarital sex she is to have with Angelo. In nearly every sense, Mariana is committing the same transgression Isabella refused to commit, and the Duke’s justification of his scheme seems somewhat legalistic, and maybe even disingenuous.