"My brother ought not to have treated me so." No; but it is for him to look to that. As for me, no matter how he behaves, I shall observe all my relations to him as I ought."
Epictetus, Discourses 3:10 (Oldfather translation)
The most annoying words that I seemed to hear constantly from Mrs. Countess, my second grade teacher, were, "Two wrongs don't make a right." Those words meant, to my childish disappointment, that I should not punch my fellow classmates in the nose every time I felt as though I had been done wrong by one of them. (And Mrs. Countess was quite right, of course!)
The ancient Stoics taught that much of the supposed injuries we receive from one another are often only imagined offences, and that, furthermore, we ultimately can only injure our own selves through Vice. But they went further than that, and (like Mrs. Countess) insisted that, while Virtue demands that we must (dispassionately) pursue justice, we must not return wrong for wrong. Another person's injustice is no justification for our own.
Nature has assigned to us all particular roles corresponding to our station in life. The brother, the son, the wife, the husband, the ruler, the subject, the citizen, etc., all have their own individualized functions. Certainly people will often fail in these roles, even doing what is entirely contrary to them, and Epictetus admits as much in the above quotation. But Epictetus reminded his students that what another person does is beyond the sphere of our control, and therefore a "thing indifferent." We must be concerned only with our own moral purpose, not the moral purpose of another. And morality consists of performing our own duties well.
Your father is neglectful. What of it? This does not prevent you from being a good (adult) son. Your husband is a difficult man; but what is stopping you from being a dutiful and faithful wife? Your neighbour is unpatriotic, unneighbourly, and quite possibly a crook; why does this mean that you have to be a bad citizen? Another pedestrian or driver is rude and abrasive; does this stop you from behaving with proper politeness and decorum while in public? The people around you are licentious and lustful like animals; but you can still be chaste and temperate as a man.If we are to be Stoics, let us let another man's sin rest with him, while we concentrate on our own.