2 Samuel 13

No man shall rely upon chastity in the past, for none whose hearts are tainted by women can become holier than David or wiser than Solomon. That none may assume that his chastity is safe in the presence of a woman related to him, let him remember how Tamar was corrupted by her brother Amnon when he pretended to be ill. Accordingly, both monks and nuns should live so chastely that they may have a good report, not only before God but also before people, and may leave to those who follow an example of sanctity.

As if to show that near relationship is no safeguard, Amnon burned with illicit passion for his sister Tamar.

This is the first introduction of divine punishment; the first but not the only one. A long series of great tribulations followed, and an almost unending succession of misfortunes scarcely left his house. Tamar was corrupted by the madness of Amnon, and Amnon was slain by Absalom. A grave crime was committed by one brother, but it was avenged more grievously by the other. In this way David, the father, was punished for the crimes of both. Two sons sinned, but three suffered for the crime of two; Tamar lost her virginity, and the loss of Absalom was mourned in Amnon. Indeed, you cannot tell for which of these two sons the loving father mourned more grievously: for him who was slain in this world by his brother's hand [Amnon] or for him who perished in the next because of killing by his own hand [Absalom].