Albert Barnes Commentary 2 Samuel 19:13

"And say ye to Amasa, Art thou not my bone and my flesh? God do so to me, and more also, if thou be not captain of the host before me continually in the room of Joab." — 2 Samuel 19:13 (ASV)

Of my bone... This phrase should be understood as in the preceding verse: are you not my bone and my flesh? It is interesting to note how the phrase is used in 2 Samuel 5:1 to refer to common descent from Israel, in 2 Samuel 19:12 for the closer kinship of the tribe of Judah, and in this verse for the even nearer kinship between David and Amasa, his sister’s son.

Captain... instead of Joab. It is very clear that David found Joab’s overbearing influence to be oppressive (Compare 2 Samuel 19:22; 2 Samuel 3:39; 2 Samuel 16:10). David was very angry with Joab at this time for killing Absalom; therefore, thinking it was of vital importance to win over Amasa and the army of Judah, he did not scruple to offer him Joab’s high post.