FORT WORTH — Minutes before she was shot and killed by a Fort Worth police officer, Atatiana K. Jefferson was playing video games in her bedroom with her 8-year-old nephew, a lawyer for her family said Sunday. Ms. Jefferson, 28, was proud of being the “cool auntie” to her siblings’ children, and had stayed up into the wee hours of Saturday morning with her nephew, Xbox controllers in their hands, according to S. Lee Merritt, the family’s lawyer. But the pair grew concerned around 2:25 a.m., he said, when they heard rustling outside the house and saw flashlights.