and he asked Amnon, “Why are you, the son of the king, so depressed morning after morning? Won’t you tell me?” Amnon replied, “I am in love with Tamar, my brother Absalom’s sister.”
We have here a particular account of the abominable wickedness of Amnon in ravishing his sister, a subject not fit to be enlarged upon nor indeed to be mentioned without blushing, that ever any man should be so vile, especially that a son of David should be so.
Commenting on 2 Samuel 13:1-20
And he said unto him, why art thou, being the king's son, lean from day today?.... Or "morning by morning", (w); he was the king's eldest son, heir to the crown, fed at his table, had everything to make him gay and cheerful, and yet pined away; his flesh wasted (x), his countenance waxed wan and pale, and especially in the mornings; in the daytime...
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 honor, even more than their father himself (see on Gen. 34:6-25).