Keisha Williams, 34, died late Thursday at Temple University Hospital. Williams, who had been in critical condition since the crash, apparently never learned her children were dead.
Two young men, Cornelius Crawford, 23, and Jonathan Rosa, 19, face a long list of charges. District Attorney Seth Williams plans to add a fourth count of murder, his office said Friday via Twitter.
Investigators said Crawford and Rosa hijacked an SUV belonging to a real estate agent on July 25, forced her into the back and sexually assaulted her. A few minutes later, Crawford, who was at the wheel, lost control of the SUV when a tire blew at the corner of Germantown and Allegheny avenues and the vehicle careened into Williams' fruitstand before hitting a tree.
Joseph Thomas Reed, 10, was killed immediately, while Keiearra Williams, 15, and Terrence Moore, 7, died a short time later. A family friend who was with them and the real estate agent were injured.
The family was trying to raise money for a children's playground planned by their church. Williams has two surviving children.
Former Philadelphia 76ers star Charles Barkley paid for the children's funeral, held Monday.
The deaths and other recent homicides in Philadelphia, including a 3-year-old girl killed by a stray bullet as she sat on a neighbor's porch getting her hair braided, have caused concern and anger in the city. Thusday night, Williams and other officials addressed a rally at Love Park near City Hall.
Organizers said the main goal of the rally was to bring people together for more effective action.
"We can play games, have these marches and these rallies, but we got go out there and do something," said Anton Moore, 28, founder and president of Unity in the Community.