Leviticus 18:9
8 You must not have sexual relations with your father’s wife; she is your father’s nakedness. 9 You must not have sexual relations with your sister, whether she is your father’s daughter or your mother’s daughter, whether she is born in the same household or born outside it; you must not have sexual relations with either of them. 10 You must not expose the nakedness of your son’s daughter or your daughter’s daughter by having sexual relations with them, because they are your own nakedness.
2 Samuel 13:13
11 As she brought them to him to eat, he grabbed her and said to her, “Come on! Get in bed with me, my sister!” 12 But she said to him, “No, my brother! Don’t humiliate me! This just isn’t done in Israel! Don’t do this foolish thing! 13 How could I ever be rid of my humiliation? And you would be considered one of the fools in Israel! Just speak to the king, for he will not withhold me from you.” 14 But he refused to listen to her. He overpowered her and humiliated her by raping her.
Notes and References
"... Marriage between a half brother and a half sister is explicitly banned by biblical law. Perhaps, it has been suggested, this prohibition was not yet held to be binding in the early tenth century, or in the royal circle in Jerusalem. But it is at least as plausible that the desperate Tamar is grasping at any possibility to buy time and deflect her sexual assailant: why do this vile thing and take me by force when you can enjoy me legitimately? ..."
Alter, Robert The Hebrew Bible: A Translation with Commentary (p. 2120) W. W. Norton & Company, 2018