Your duties depend on your relationships. Is someone your father? Then you should care for him, respect him, and take his criticism patiently. But what if he's a bad father? Your natural duty isn't to a good father — it's to a father, period. Is your brother unfair? Keep being fair to him anyway. Don't focus on what he does. Focus on what you need to do to keep your character in line with nature. Another person can't hurt you unless you let them. You only get hurt when you choose to be hurt. Practice this way of thinking with all your relationships — neighbor, fellow citizen, boss. You'll know what your duties are in each case.
Duties are universally measured by relations. Is a certain man your father? In this are implied taking care of him, submitting to him in all things, patiently receiving his reproaches, his correction. But he is a bad father. Is your natural tie, then, to a _good_ father? No, but to a father. Is a brother unjust? Well, preserve your own just relation toward him. Consider not what _he_ does, but what _you_ are to do to keep your own will in a state conformable to nature, for another cannot hurt you unless you please. You will then be hurt when you consent to be hurt. In this manner, therefore, if you accustom yourself to contemplate the relations of neighbor, citizen, commander, you can deduce from each the corresponding duties.