Amnon plans to rape Tamar
Amnon fell in love with his half-sister Tamar. His friend Jonadab advised him to act like he was sick and ask for Tamar to take care of him.
Key passages
2 Sa 13:1–5

It happened afterwards that Absalom the son of David had a beautiful sister whose name was Tamar, and Amnon the son of David fell in love with her. And Amnon was so frustrated that he felt ill because of Tamar his sister, because she was a virgin, and it was …