2 Samuel 13:1 In the course of time, Amnon son of David fell in love with Tamar, the beautiful sister of Absalom son of David.

2 Amnon became so obsessed with his sister Tamar that he made himself ill. She was a virgin, and it seemed impossible for him to do anything to her.

3 Now Amnon had an adviser named Jonadab son of Shimeah, David’s brother. Jonadab was a very shrewd man. 4 He asked Amnon, “Why do you, the king’s son, look so haggard morning after morning? Won’t you tell me?”

Amnon said to him, “I’m in love with Tamar, my brother Absalom’s sister.”

I can't tell these people apart without some commentary. They are all cousins, brothers. sisters, and uncles.

This is from blueletterbible.org.

Absalom the son of David had a lovely sister, whose name was Tamar: This brother and sister were the children of David through his wife Maacah, who was the daughter of Talmai, king of Geshur (2 Samuel 3:3).

Amnon the son of David: Amnon was David’s first born son, born from his wife Ahinoam the Jezreelitess (2 Samuel 3:2). Being the first born, Amnon was the crown prince – first in line for the throne of Israel.

Amnon the son of David loved her: Amnon longed for Tamar so much that he became lovesick. It was even more difficult for him because she was a virgin – meaning that she was available for marriage, but not to Amnon because marriage between half-brother and half-sister was forbidden.

Now... I have to remember... whatever happens in this story can be traced right back to David and the murder of Bathsheba's husband Uriah. In 2 Samuel 12, the prophet Nathan warned David that "calamity" was on it's way.

2 Samuel 12:11 “This is what the Lord says: ‘Out of your own household I am going to bring calamity on you. Before your very eyes I will take your wives and give them to one who is close to you, and he will sleep with your wives in broad daylight. 12 You did it in secret, but I will do this thing in broad daylight before all Israel.’”

This is from Bible.org.

If you are like me, you almost find your head swimming when you read this account about David's relatives. Characters in this plot include David; Jonadab, David's nephew, son of David's third brother, Shimeah; Amnon, David's first-born son of Ahinoam; and Tamar and Absalom, daughter and son of Maacah, David's third wife (who was the daughter of Talmai, the king of Geshur). It is very difficult to remember who belongs to whom, isn't it?

Amnon has a very serious problem. He has “fallen in love” with his beautiful half-sister, Tamar. According to the Law of Moses, there is no way he could have this woman as his wife.

Amnon was so frustrated because of his sister Tamar that he made himself ill, for she was a virgin, and it seemed hard to Amnon to do anything to her.

Amnon makes himself ill because she is a virgin, and because of this, he “was not able to do anything to her.” We are not told that Amnon loves Tamar and wants to marry her. I believe we are told that Amnon wants to have sex with Tamar, but she is a virgin, and that staying so until marriage is her commitment. Amnon wants to have a sexual relationship with Tamar. He is willing, but she is not. She is a virgin and seems intent on keeping it this way. No wonder he could not get anywhere with her. And no wonder he is frustrated. His frustration has gotten to the point that it is making him ill (lovesick?). The symptoms of this “illness” are not stated, but I would imagine possible symptoms would be an upset stomach, a lack of appetite, and lack of sleep.

This might be a good time to bring up the fact that masturbation could be deadly. In Genesis 38:9-10 the fate of Onan is death. Onan didn't want to get his brother's widow pregnant so he "spilled his seed".... masturbation was looked upon the same way. So if Amnon was not spilling his seed to avoid death and Tamar was driving him crazy [willingly or unwillingly] then he probably had a raging case of epididymal hypertension.

No doubt the calamity is boiling.