A cousin, arriving at the two-storey semi-detached house a short time later threw herself to the ground outside in tears.
As she screamed, gardai at the scene attempted to comfort her.
"It was a dreadful scene," said local GP and former Government Minister Jim McDaid who was called to the scene.
"It was horrific to see the result of what was clearly a very intense fire which burned in both the living room and the bedroom above where the girls were.
"It is a terrible, terrible tragedy for the family," Dr McDaid said.
Parish Priest Fr Brian Quinn said the scene was "traumatic."
Garda forensic investigators spent the day examining the house after four fire tenders left the area at lunchtime.
While investigations are ongoing, the incident is not being treated as suspicious.
They were concentrating their investigation in the living room at the front of the house, where a Christmas tree had been and the bedroom immediately above where the two women perished.
Neighbours have expressed shock at the incident.
"We can't believe this has happened here," one said. "They were two lovely girls, full of life."
Local councillor and former Letterkenny mayor Gerry McMonagle, who knows the Brogan family, said the entire town had been left "in grief" after the deaths.
"I've known the family for years and they are devastated," he said.
"I don't think it has even sunk in yet but it will over the next few days and people will be more distraught." Friends of the young women, both former pupils at Errigal College, left hundreds of messages on their Facebook pages last night. One read: "Such beautiful girls. Heaven has gained two angels."
Jody had posted a message on her Facebook page just a few days ago to welcome in the New Year.
It read: "To all who have lost someone, gained someone and who have yet to become someone, may 2014 be a better year for all."
Gardai have appealed for anyone with information on the incident to contact officers at Letterkenny Garda Station on 074 916 7100.