Tricky question

This is a question from one of the major biology textbooks. I've always felt questions like this one to be right on the borderline between being a trick question and not. What do you think? Fair question? I'll post a comment on what the answer is NOT.

Karen and Steve each have a sibling with sickle-cell disease. Neither Karen nor Steve nor any of their parents have the disease, and none of them have been tested to reveal the sickle-cell trait. Based on this incomplete information, calculate the probability that if this couple has a child, the child will have sickle-cell disease.