2 Samuel 13:3

But Amnon had a friend, whose name was Jonadab, the son of Shimeah David's brother: and Jonadab was a very subtil man.

Jonadab, the son of Shimeah - or Shammah (1 Samuel 16:9), who was one of David's brothers. Although none of them were promoted to places of honour and emolument under government, probably from the feelings of alienation that subsisted between the king and his brethren, David seems to have acted in a kindly spirit toward their children; and the case of Jonadab is one of several known instances in which he had these young relatives about his court. By the counsel and contrivance of this scheming cousin a plan was devised for obtaining an unrestricted interview with the object of his attachment.

My brother Absalom's sister. In Eastern countries, where polygamy prevails, the girls are considered to be under the special care and protection of their uterine brother, who is the guardian of their interests and their honour, even more than their father himself (see the notes at Genesis 34:6).