I think John knows enough of honor and principle to feed a nagging conscious about the neglect of his sisters but at the same time we see he has not the will to do what he knows he ought. John goes full tilt at Elinor to assure she understands his poverty to excuse further inattention on his part. If he does attempt to rise above himself, as in inviting them to stay with them (Ch. 36), Fanny’s puts an end to it by playing on his weaknesses just as she put an end to his helping them in Ch. 2. I think this rather weak duel we see in John and his continual capitulation to Fanny (“a strong caricature of himself; more narrow-minded and selfish”) reinforces to me the fact that “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” (Ch. 1). The good effect a spouse can have on their partner is brought up again by Col Brandon when he thought Marianne was to wed Willoughby, he told Elinor “sometimes I thought your sister's influence might yet reclaim him [Willoughby]” (Ch. 31). Considering his behavior Marianne had no good effect on Willoughby and, as we saw in her joining him in disparaging Col Brandon (Ch. 10), he had no good effect on her either. (:D)