Fire crews were called to the house on Callanish at 1630 BST on Saturday after the blaze was discovered in a first floor bedroom.
Ruth MacIver was rescued from the house by firefighters then taken to Western Isles Hospital in Stornoway, where she was pronounced dead upon arrival.
Her 60-year-old sister, Morag Stewart, is still in hospital, receiving treatment for smoke inhalation.
A forensic examination, to be carried out by the police and the fire service, is due to begin on Monday.
However, Northern Constabulary said the cause of the fire was not thought to be suspicious.
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