Cordail Kuhns was pronounced dead Saturday afternoon at Crozer-Chester Medical Center outside Philadelphia, according to hospital spokesman Grant Gegwich.
Cordail's mother, father and three siblings were also killed by the fire, The Intelligencer Journal/Lancaster New Era reported.
Crystal Kuhns, 41, died Friday morning, and her daughter Mickey, 4, died Thursday evening, Gegwich said. Authorities previously identified three other victims as David Kuhns, 51; Skylar Kuhns, 8; and Shawn Kuhns, 6.
Sixty-five-year-old homeowner Jimmie Moore was also killed in the blaze, authorities said.
Fire officials said the blaze started in the kitchen of a three-story duplex in Lancaster at about 3 a.m. Thursday after frozen potatoes were left in the oven.
Fourteen members of several families lived in the home, which did not have smoke detectors, fire chief Timothy Gregg said.
The Lancaster newspaper reported Friday that Moore's pregnant daughter, Martha Moore, 22, escaped through a second-floor window and later had an emergency cesarean section Thursday morning, giving birth to 4-pound, 5-ounce daughter, Zaliah.