The record of Amnon raping his sister is in 2 Sam. 13:1-22. Basically, Amnon falls in love with Tamar, and his desire grows until he concocts a plan to try to have sex with her. Tamar is a better person than Amnon, and she resists the seduction, so Amnon adds more sin to his list and rapes her. It’s a terrible set of events. And as soon as Amnon has abused Tamar he now hates her! 2 Sam. 13:14-15 says:
But he would not listen to her, and being stronger than she, he violated her and lay with her. Then Amnon hated her with very great hatred, so that the hatred with which he hated her was greater than the love with which he had loved her. And Amnon said to her, “Get up! Go!”
Once Amnon had unleashed the lust that had been festering inside him he hated Tamar — the cloud of his lust dissipated, and his “love” had gone. If only he’d not let his lust confuse and fog the situation in the first place!
It’s a shocking example of how horribly our lusts can take control of us if we indulge them. It’s also an example of how we can confuse lust for love — sometimes we can think we are in love with someone, but really we are just lusting after them.