I think John Dashwood has a skewed image of himself. He does not see himself as selfish and cold-hearted and really believes himself capable of generosity in the fulfillment of his promise to his father. I think one beauty of Fanny’s attack on John’s plans to augment his sister’s fortunes or his step-mother’s income is that she minimizes their needs into nothing convincing him his plan is over generous to a fault. In his mind helping to move their possessions and the occasional neighborly gifts he envisions is a kindness to women who really have no want.

He really felt conscientiously vexed on the occasion; for the very exertion to which he had limited the performance of his promise to his father was by this arrangement rendered impracticable. (Ch. 5)

John is only “conscientiously vexed” at not being able to fulfill his promise to his father as planned rather than an idea that by not doing so he is neglecting his step-family. I really think John’s complaining about the “the increasing expenses of housekeeping, and of the perpetual demands upon his purse” in Ch. 5 attests to how dearly he embraced Fanny’s opinion in Ch. 2 that “They will be much more able to give you something” and is a belief he has already done enough to satisfy his promise. John bids farewell to his step-mother and sisters with his self-image intact but I think Henry Dashwood knew his son’s cold-heartedness and selfishness very well and that is why he extracted the promise from John in the first place—I think he did suppose his son would neglect his step-family and he was right. (:D)