Tamar's claim was a true statement, as there was no legal impediment to Tamar marrying Amnon. This position subdivides regarding the nature of Amnon and Tamar's family ties and why they were permitted to each other:
No Blood Relationship
Amnon and Tamar were not related at all.
Tamar's Relationship to Avshalom – Tamar was born from the same mother as Avshalom (Ma'akhah), but a different father, making them half-siblings.
1 Tamar's Relationship to Amnon – Tamar and Amnon were not related at all, sharing neither a father nor a mother. As such, they were totally permitted to each other.
"תָּמָר אֲחֹתִי" – Even though there were no blood ties between Amnon and Tamar, since Tamar grew up in the palace with all the other children of David, she was treated and referred to as a sister.
"וַיִּפָּלֵא בְּעֵינֵי אַמְנוֹן לַעֲשׂוֹת לָהּ מְאוּמָה" – Abarbanel questions that if Tamar was permitted to Amnon why could he not think on his own to ask to wed her? Why was he at a loss as to how to touch her? The Rid suggests that Amnon was never interested in marriage, only in sleeping with Tamar. He could not figure out what to do, since as a virgin, she was very modest and rarely went out alone.
Yonadav's intentions - Prevent rape – Itai Elitzur2 suggests that Yonadav was actually trying to prevent a potential rape.3 Recognizing that Amnon was not interested in marriage, he suggested that Amnon speak to his father, assuming that David would recognize the situation and intervene (by either having the two wed, or preventing the situation altogether.) Yonadav therefore told Amnon to ask David, "תָּבֹא נָא תָמָר אֲחוֹתִי וְתַבְרֵנִי לֶחֶם", a euphemism for relations. Amnon, not knowing Yonadav's intentions, instead asked that Tamar prepare "לְבִבוֹת". As such, David missed the hint.
- Facilitate an innocent meeting – Alternatively, it is possible that, knowing that Amnon could marry Tamar, Yonadav never thought that Amnon would rape her. He thought Amnon was simply looking for a way to meet Tamar, who, due to her virgin status, was not easily approachable. He therefore found a way to facilitate such a meeting.
"אַל תַּעֲשֵׂה אֶת הַנְּבָלָה הַזֹּאת" – The abomination that Tamar was referring to was Amnon's desire to sleep with her like a prostitute rather than wed her. Presumably, if they had really been siblings, she would not have sufficed with saying that the act was an abomination, but clarified that it was actively prohibited as incest.
Why didn't Amnon agree to Tamar's suggestion? The Rid points out that Amnon never had any intention of marrying Tamar, and only wanted to fill his desire, so her pleas fell on deaf ears.
Avshalom's reply: "הַחֲרִישִׁי אָחִיךְ הוּא" – Avshalom's advice that Tamar be silent since Amnon was "her brother" is somewhat difficult considering that they were not in fact related. It is possible that Avshalom was simply saying that this was a family affair and should not be publicized outside the palace walls.
Not Legally Siblings
Though Amnon and Tamar were biologically related, they were not legally considered siblings.
Tamar's Relationship to Avshalom – Avshalom and Tamar were full siblings, sharing both a mother (Ma'akhah) and father (David). This is why she is first introduced in relationship to him rather than Amnon (לְאַבְשָׁלוֹם בֶּן דָּוִד אָחוֹת יָפָה וּשְׁמָהּ תָּמָר), and why Amnon similarly tells Yonadav, "תָּמָר אֲחוֹת אַבְשָׁלֹם אָחִי אֲנִי אֹהֵב".
Ma'akhah – According to all these sources, Ma'akhah was a beautiful captive woman (אשת יפת תואר)
4 who bore Tamar from relations with David prior to her conversion.
5 As such, legally Tamar was not considered a daughter of David but rather that of a Gentile maidservant.
6 Tamar's Relationship to Amnon – Though David fathered both Amnon and Tamar, since Tamar was conceived before Ma'akhah converted, she was Amnon's biological, but not halakhic, half-sister and not prohibited to him.
7 "תָּמָר אֲחֹתִי" – Tamar is referred to as Amnon's sister throughout the story, because, despite her legal status, she was nonetheless David's daughter.
Yonadav's intentions – Malbim
8 claims that Yonadav, knowing that Amnon could legitimately marry Tamar, had never intended that he take her by force. He suggested that Amnon fake illness and talk to his father as a way of alerting David to Amnon's desire for Tamar, assuming that David would then suggest that they marry.
9 Moreover, it would provide an opportunity for Amnon to let Tamar herself know of his intentions.
"אַל תַּעֲשֵׂה אֶת הַנְּבָלָה הַזֹּאת" – Tamar's goal in all her pleas was that Amnon not treat her as a prostitute but rather legitimately wed her. Ralbag points out that Tamar never uses the argument that the act would be transgressing the prohibition of גילוי עריות, only "that this is not done in Israel".
Why didn't Amnon agree to Tamar's suggestion? Since Amnon was overcome by physical desire, he had no interest and no self control to wait to approach David.
Amnon's hatred – Malbim suggests that Amnon had never loved Tamar, just lusted after her. Thus as soon as his desire was satiated, he recognized that he had done an abominable act but instead of hating himself for his actions, he projected that hatred onto Tamar.
10 Tamar knew that David would never approve of the marriage, but was grasping at straws hoping that her words would convince Amnon to release her.
Tamar's Relationship to Avshalom and Amnon – Avshalom and Tamar were full siblings, while Amnon and Tamar were half siblings, related through David, but not through their mothers..
"תָּמָר אֲחֹתִי" / "תָּמָר אֲחוֹת אַבְשָׁלֹם" – At times Tamar is referred to as Amnon's sister and at times as Avshalom's sister because she was in fact related to both of them. Abarbanel suggests that in the opening of the story she is specifically connected to Avshalom since only they were full siblings and it was this relationship which led to Avshalom's later desire for revenge. Similarly, when Amnon approaches Yonadav he specifies "תָּמָר אֲחוֹת אַבְשָׁלֹם" to disambiguate her from Avshalom's daughter of the same name.
11 "וַיִּפָּלֵא בְּעֵינֵי אַמְנוֹן לַעֲשׂוֹת לָהּ מְאוּמָה" – Amnon did not know what to do because he recognized that, being his sister, Tamar was off limits.
Yonadav's advice – Abarbanel maintains that Yonadav was "חכם להרע" and did not attempt to keep Amnon from sinning, but rather to facilitate his wrongdoing and find a way for him to fill his desires.
"אַל אָחִי אַל תְּעַנֵּנִי" – Abarbanel suggests that in the words, "אַל אָחִי", Tamar was hinting to Amnon that he should not do the act specifically because they were brother and sister. The "abomination" that is "not done in Israel" is the fact that these relations would constitute גילוי עריות, one of the prohibited acts of sexual intercourse.
"כִּי לֹא יִמְנָעֵנִי מִמֶּךָּ" – Abarbanel maintains that Tamar was suggesting to Amnon that since he was love sick to the point of death, and nothing stands before saving a life, David would surely approve of the marriage despite the prohibition inherent in the union.
Goal of Tamar's plea – Tamar was simply trying to push Amnon off, hoping he would wait to talk to David, giving her means of escape. She did not truly believe, however, that David would agree.
Amnon's refusal – Amnon was not convinced by Tamar's lie, recognizing it for the ruse it was.
Tamar was unaware of the law, and truly believed that David would allow the marriage.
Tamar's Relationship to Avshalom and Amnon – While Tamar and Avshalom were full siblings, Amnon and Tamar were only half-siblings, sharing a father (David) but not their mother.
"תָּמָר אֲחֹתִי" / "תָּמָר אֲחוֹת אַבְשָׁלֹם" – Tamar is referred to as the sister of both Avshalom and Amnon because she was related to both.
"וַיִּפָּלֵא בְּעֵינֵי אַמְנוֹן לַעֲשׂוֹת לָהּ מְאוּמָה" – Amnon was at a loss of how to approach Tamar because she was prohibited to him, and moreover, being a virgin, she did not often venture outside.
"אַל תַּעֲשֵׂה אֶת הַנְּבָלָה הַזֹּאת" – Tamar was not referring to the prohibition of גילוי עריות and incest, as she thought that she and Amnon would be permitted one to another, but rather to Amnon sleeping with her before marriage.
Why didn't Tamar know that marriage would be prohibited? - Unlearned – Ibn Kaspi suggests that Tamar might simply not have been learned in the law ("כי באמת לא למדה התלמוד והגמרא").
- Custom of the time – Hoil Moshe adds that it is likely that other cultures of the time looked only at the maternal line to determine prohibited relationships, while allowing marriages of half-siblings through the father. Even Avraham seems to make this distinction, telling Avimelekh that Sarah was "אֲחֹתִי בַת אָבִי הִוא אַךְ לֹא בַת אִמִּי וַתְּהִי לִי לְאִשָּׁה".
Amnon's refusal – In contrast to Tamar, Amnon knew very well that David would never permit the marriage, leading him to the alternative of rape.