Absalom the son of David had a fair sister, whose name was Tamar. (13:1a)
Maachah was Absalom's mother (2 Samuel 3:3). Tamar is the only named daughter of David, and is Absalom's full sister.
Amnon (13:1b)
Amnon was David's firstborn son (2 Samuel 3:2), and would, therefore, be expected to become king when his father died.
Tamar remained desolate in her Absalom's house for the rest of her life. (13:20)
Since Tamar was no longer a virgin, she could not get married. (Deuteronomy 22:13-21)
At the party, Absalom said to his servants, "Kill Amnon". (13:28)
Absalom was David's third son. The second son was Chileab (or Daniel if you believe 1 Chronicles). But he isn't mentioned again after his birth. Maybe he died before David.
So it looks like Absalom is now the crown prince after killing his older brother Amnon.