Tatiana Meakens, 23, and Britany Meakens, 22, left four children unattended in the home, which was heated by space heaters and a hot plate, prosecutors said, while the two attended separate parties.
The dead children were identified by the Cook County medical examiner's office as Javaris Meakens, 2, and Jariyah Meakens, 3. They died early Saturday.
Police said Tatiana was their mother; Britany was their aunt. Two other children managed to run out of the apartment and survived.
Cook County Assistant States Attorney Jamie Dickler said the two women agreed Tatiana would watch them so Britany could attend a housewarming party Friday, the Chicago Tribune reported.
But Dickler said Tatiana left to go to a CD release party in Blue Island after getting a phone call. She only returned hours later after being told of the fire.
A defense lawyer said their landlord had turned off the heat, the Tribune reported.
Autopsies Sunday showed the children died of carbon monoxide intoxication and smoke and soot inhalation. The Cook County medical examiner's office ruled the deaths were accidental.
Authorities said two boys, Darnell Meakens, 7, and Marquis Meakens, 4, believed to be the sons of Tatiana Meakens, escaped the blaze and were placed in the care of state child protective services.
Authorities said no adults were home when the fire began about 3:30 a.m. A hot plate being used to heat the apartment was being investigated as a possible cause of the fire.