I like the description of Fanny eroding John’s resolve to honor his promise. (:D) I think one reason Fanny persuades rather than demands may be to maintain the appearance that he rather than she is the guiding force in their marriage. John thinks well of himself and his ideas and for Fanny to dismiss them outright might not go over well. Another reason might be that it is unnecessary to disturb their domestic tranquility with demands. John is easily persuaded to give up his notions of what is right for hers. As you said, Fanny makes him comfortable with the idea of doing practically nothing for his step-family so he is happy and she gets what she wants. (:D)