Hollywood takes up a social issue, hard data and clarity can be elusive. My Sister’s Keeper, a drama in theaters this summer, provides a handy example of Tinseltown’s tendency to let tears trump truth. Told through a series of flashbacks, the plot is set in motion by a medical specialist’s shocking proposal to a mother desperate to save her gravely ill child: Create a genetically engineered sibling who can donate bone marrow and other vital tissue to cheat death. That suggestion would stop most parents in their tracks. But this mom grabs hold of the idea and runs with it....