I can't help but feel that Mr. Dashwood believed that John's relationship with his step-mother was favorable. When John agrees to assist Mrs. Dashwood and his sisters, Mr. Dashwood "was rendered easy by such assurance". Surely after having her as a daughter-in-law for over four years, Mr. Dashwood knew a little of Fanny's character to suspect that she was greedy and capable of manipulating John. Yet, Mr. Dashwood had enough confidence in John's respect for Mrs. Dashwood to feel that mother and daughters would be financially protected under John's care. His son's word wasn't accepted warily but with full belief that John would act on his promise in such a way that even Fanny's being "more narrow-minded and selfish" couldn't weaken John's resolve to care for his father's other family.