I think a common thread in Austen is the affect spouses have on each other. John Dashwood (Ch. 1) married a “strong caricature of himself” and her cold-heartedness, insensitivity, and selfishness only enhance the same qualities in him. John was affected by his father’s request to help his sisters and stepmother but his promise could not withstand Fanny’s assault—her arguments (Ch. 2) were too reflective of his own heart to be ignored. I think John is weak-willed. The narrator says had “he married a more amiable woman, he might have been made still more respectable than he was; he might even have been made amiable himself.” which suggests that instead of his promise being ground into nothing by Fanny another wife could have encouraged him to honor it and he would have. I think Fanny has the stronger personality and coupled with the facts they are of like mind and he is “very fond of his wife” she has a fairly easy time influencing him to do as she wishes but IMO he is still responsible for his choice.