2 Samuel 13:1
And it came to pass after this, that Absalom the son of David had a fair sister, whose name was Tamar; and Amnon the son of David loved her. And it came to pass after this, that Absalom the son of David had a fair sister, whose name was Tamar; and Amnon the son of David loved her.
This opening begins the account of Amnon's crime against Tamar that leads to Absalom's revenge in killing Amnon. Amnon was David's first-born, and Tamar was the fair sister of Absalom, and Amnon's act against her was a shameful, incestuous and unnatural adultery upon which the law pronounced death, doubly guilty because it was both incestuous and unnatural. David permitted the crime to pass unpunished and unrebuked, self-condemned by his own sin and failing to bring the offender to justice.
Absalom, as the natural protector of his foully wronged sister, concealed his purpose of revenge for two full years only to strike more surely, having the drunken Amnon slain by his command at a feast of the king's sons. These events brought home to David the consequences of his own wrongdoing against Uriah and Bath-sheba. The slaying of Amnon meted out twofold judgment upon David under his own fourfold sentence.