Moses McKnight, 4, was injured in the fire that erupted Wednesday morning in the 900 block of East 15th Street.
He died just before 3:00 p.m. Friday, Action News has learned.
Moses' sister, 6-year-old Bella McKnight and their cousin, 9-year-old Amajhay Ferguson, also died in the fire.
There were six children in the house at the time of the blaze. Three of them live in the home and the other three were cousins staying for a visit.
The father of the kids, Timothy Ferguson, managed to get his wife and three children out safely.
Five firefighters were also hurt in the blaze, which happened two blocks from the Widener University campus.
The cause of the fire remains under investigation.