In "The Ransom of Red Chief," why does Bill ask Sam the following?: "There isn't any heart disease in your family, is there?"

Actually it is Sam who asks Bill the question. Here is the pertinent dialogue from the story.

“Bill,” says I, “there isn’t any heart disease in your family, is there?”
“No,” says Bill, “nothing chronic except malaria and accidents. Why?”
“Then you might turn around,” says I, “and have a look behind you.”
Bill turns and sees the boy, and loses his complexion and sits down plump on the ground and begins to pluck aimlessly at grass and little sticks.

Bill is the one who is stuck with Red Chief most of the time. He thought he had gotten rid of the boy by treating him roughly and sending him back home to his father. But Sam sees the boy returning from behind Bill and is a little afraid of alarming him by warning him of Red Chief's approach. At the same time he feels he needs to warn him because they never know what the boy might be planning to do next. That is why he asks, in a semi-humorous fashion, whether Bill has any heart disease in his family. He thinks Bill might have a fatal reaction to the shock. The biggest problem these would-be kidnappers have in the entire story is controlling their victim. Red Chief makes victims out of Bill and Sam, especially out of Bill, who has to act as custodian while Sam is negotiating with the boy's father and running other errands.

A very short while later they receive a reply to their ransom letter. Not only are they stuck with Red Chief but his father wants them to pay him $250 for taking the boy off their hands. This is the ironic situational twist on which the story is built. Bill and Sam have had so much trouble with their victim that, as Ebenezer Dorset expected, they are willing to accept his terms.