Was it not a sin for Tamar and Amnon to get married because they were from the same father, even though they had sex?
A difficulty arises when multiple laws are violated at the same time and penalties for these different laws vary.
- Amnon lusted after an unmarried woman, which violated the teaching, "The righteousness of the upright will deliver them, but the unfaithful will be caught by their lust" (Proverbs 11:6).
- Amnon had sex with his sister, which violated, "The nakedness of your sister, the daughter of your father, or the daughter of your mother, whether born at home or elsewhere, their nakedness you shall not uncover" (Leviticus 18:9). The consequence of this law is "If a man takes his sister, his father's daughter or his mother's daughter, and sees her nakedness and she sees his nakedness, it is a wicked thing. And they shall be cut off in the sight of their people. He has uncovered his sister's nakedness. He shall bear his guilt" Leviticus 20:17).
- Amnon raped an unmarried woman, which violated, "If a man finds a young woman who is a virgin, who is not betrothed, and he seizes her and lies with her, and they are found out, then the man who lay with her shall give to the young woman's father fifty shekels of silver, and she shall be his wife because he has humbled her; he shall not be permitted to divorce her all his days" (Deuteronomy 22:28-29). This passage is broader than just rape, "seized" carries the idea of enticing in Hebrew, but it can be applied to force sex as well.
- Amnon refused to marry Tamar, which was the consequence of having sex with her, and thus broke Deuteronomy 22:28-29 in a different way. It wasn't his decision under the Law of Moses.
So applying all the laws being broken:
- Amnon would be forced to pay 50 shekels for Tamar's dowry.
- If David allowed (Exodus 22:16-17), he would have to marry Tamar with no option for divorce.
- Amnon would no longer be considered an Israelite, regardless of whether he was forced to marry Tamar or not.
One of the sad points of this story is that these laws were not upheld. This is why Absalom thought he was justified to murder his brother.