Charles Ellicott Commentary 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." — 2 Samuel 13:1 (ASV)

It came to pass after this. —This formula applies to the narrative which follows as a whole: not, of course, to the fact immediately afterwards mentioned, that Absalom’s sister was Tamar. This may illustrate the use of the same phrase in other places.

Absalom and Tamar were children of Maacah, daughter of Talmai, king of Geshur, and the former, at least, had been born during David’s reign at Hebron (2 Samuel 3:3). It is probable that the events here narrated occurred soon after the war with the Ammonites and David’s marriage with Bath-sheba.

Amnon was David’s first-born son (2 Samuel 3:2).