A 3-year-old Petaluma girl was accidentally shot and killed by her 4-year-old brother while the two were playing, police said. Erica Nicholson was shot in the head with a .38-caliber revolver her brother, Eric, had taken from a closet while their mother slept, Sgt. Dave Dohn said. "While he was playing with the firearm, it accidentally discharged," Dohn said. The children's mother called paramedics, who administered cardiopulmonary resuscitation and took the girl to Petaluma Valley Hospital, where she was pronounced dead. The shooting occurred after the boy climbed on a chair and took the loaded gun from a closet shelf. The children had been playing in a bedroom they shared while the mother slept on a living room couch. "The mother said the boy knew where she kept the gun," Dohn said.