Actually myotonic dystrophy is a dominant inherited disorder which means if one parent has it then there is a 50% chance that a child will inherit it. If it turns out Steve hasn't inherited it there's no chance that any of he's children will. For Jim to have inherited it one of Jim's parents must have had it. You can only be a carrier of a particular allele (variation of a gene, my lecturers wouldn't be very happy if I used the wrong term 😛) if it is recessive, which this disease is not.