A 45-year-old wheelchair-bound woman who watched her husband gun down their 6-year-old child and the woman’s 74-year-old mother at their home near Volos in central Greece last week has died, authorities said yesterday. Christina Petra had been suffering from a rare muscle-wasting disease and was feeling unwell yesterday morning. She was taken to the hospital in Volos but died soon afterward. Doctors said Petra suffered heart failure. She is to be buried today. The couple’s other child, an 8-year-old boy, is being treated in a hospital in Athens for injuries he suffered during his 46-year-old father’s shooting spree last Thursday. The father, Michalis Psaris, who has been in a hospital in Volos after suffering self-inflicted gunshot wounds, was visited by a prosecutor and magistrate yesterday so he could give his testimony.