Wiltshire Police said the blaze at a house in Swindon on Sunday night was a "tragic accident" and the death is not being treated as suspicious.
The cause of the fire is still being determined by fire investigators.
Two adults and a 16-month-old baby are still in a serious but stable condition in hospital.
Emergency crews were called just before midnight to the terraced house in Manchester Road.
A boy aged 13 and a 23-year-old woman were also injured in the fire but were not thought to be in a critical condition.
A 30-year-old woman seriously injured in a severe house fire has died in hospital.