There are actually two Tamars in the Old Testament (or Tanach). The other one was a daughter of David. Her half-brother Amnon became obsessed with her, and at the advice of his cousin Jonadab, he feigned illness and asked for Tamar, and when she came, raped her. In consequence, another brother, Absalom, tricked Amnon to coming to a feast a year later at which he murdered him. The story of this Tamar may be found in 2 Samuel 13. (click to read).