Fulfill your role
Principle 30 from the Enchiridion
Keep your standards even when they neglect theirs. Other people's behavior does not remove the responsibility that still belongs to your part.
Original Passage
Duties are universally measured by relations. Is anyone a father? If so, it is implied that the children should take care of him, submit to him in everything, patiently listen to his reproaches, his correction. But he is a bad father. Are you naturally entitled, then, to a good father? No, only to a father. Is a brother unjust? Well, keep your own situation towards him. Consider not what he does, but what you are to do to keep your own faculty of choice in a state conformable to nature. For another will not hurt you unless you please. You will then be hurt when you think you are hurt. In this manner, therefore, you will find, from the idea of a neighbor, a citizen, a general, the corresponding duties if you accustom yourself to contemplate the several relations.
Modern Interpretation
Epictetus holds that every role carries some responsibility. As a parent, child, sibling, friend, or citizen, our first task is not to control the other person but to look after our own part.
Someone else's bad behavior does not excuse ours. If they are unfair or rude, we can still decide whether to answer in kind or keep our own standards.
A more useful question is, "What is my part in this relationship?" rather than only, "Who started it?" We cannot choose the other person's conduct, but we can choose whether it determines ours.
In Practice
A family member speaks harshly during an argument. Your first impulse is to match their tone.
Ask what your role requires from you. You can be firm without insulting them and set a boundary without pretending their behavior was acceptable.
Reflection Question
In a difficult relationship, what still belongs to your part even when the other person neglects theirs?