Why was Michael's sister sick?

Michael's baby sister Joy—as she comes to be known—was born prematurely. Like a lot of premature babies, she's experienced serious health problems. Joy is gravely ill; she has a heart condition which is so serious that it's thought unlikely that she'll survive for much longer. To make matters worse, none of the doctors at the hospital seem to display more than superficial concern for Joy's parlous condition. It's no wonder that Michael christens Dr. Dan as Dr. Death. Dr. Dan's pale skin gives him the appearance of a harbinger of doom, a bringer of death. His whole demeanor eerily foreshadows what appears to be Joy's imminent demise. In fact, the doctor's lack of empathy is a contributory factor in Joy's clinical death on the operating table. Thankfully, however, the mysterious Skellig's on hand to put things right.