Albert Barnes Commentary 2 Samuel 13:5

"And Jonadab said unto him, Lay thee down on thy bed, and feign thyself sick: and when thy father cometh to see thee, say unto him, Let my sister Tamar come, I pray thee, and give me bread to eat, and dress the food in my sight, that I may see it, and eat it from her hand." — 2 Samuel 13:5 (ASV)

Make yourself sick - “Pretend you are ill.” (Compare 2 Samuel 14:2.)

That I may see it - He was to pretend that he had no appetite for anything that came from the kitchen, but that if he saw it cooked, he would be able to eat it.