David was angry, but did nothing. Amnon thought he got away with it (and for a while he did). Absolom knew David was not following the law in dealing with it, nor taking responsibility to parent.
Passive parenting does not solve problems.
2 Samuel 13:20-22 ESV
[20] And her brother Absalom said to her, “Has Amnon your brother been with you? Now hold your peace, my sister. He is your brother; do not take this to heart.” So Tamar lived, a desolate woman, in her brother Absalom’s house. [21] When King David heard of all these things, he was very angry. [22] But Absalom spoke to Amnon neither good nor bad, for Absalom hated Amnon, because he had violated his sister Tamar.
