The brief answer is that Sam does not want Bill to have a heart attack (or at least he is acting as if he is worried that Bill will have a heart attack). So he asks Bill if anyone in his family has had heart disease before he tells Bill to turn around.
The point of this line is that "Red Chief" has been torturing Bill ever since Sam and Bill kidnapped him. He has bitten and kicked Bill and made him eat grass, among other things. At the point in the story where Sam asks about heart disease, Bill thinks that they have finally gotten rid of the kid. But they have not -- Red Chief is right behind Bill.
Sam knows this will not make Bill happy and that fact is emphasized by Sam asking Bill about the heart disease. The author has put this line in here to be funny and to stress how much Red Chief has made Bill's life miserable.