Shannon Gallagher, who was 15, had wrapped her Christmas presents and written a note to her mother a short time before her body was found on December 12, 2012.
Her 13-year-old sister Erin had taken her own life six weeks earlier. Details of Shannon's last hours were given at an inquest in Letterkenny yesterday.
Among those who attended were Shannon's mother Lorraine and grandfather John, as well as her foster parents Gerard and Karen Doherty.
Mr and Mrs Doherty gave details of finding Shannon dead at their home at Stranamuck, Castlefin, Co Donegal, at about 11.40pm.
The inquest was told that Shannon had agreed to move in with a foster family as she and her mother had been "arguing a lot" following Erin's death.
Mrs Doherty told the inquest she had found Shannon to be a normal teenager who mixed well with her family after moving in on November 30.
She recalled how Shannon often spoke about Erin and how important it was not to be influenced by other people.
Mrs Doherty said that, on the day she died, Shannon had been watching videos on her phone of her late sister but showed no signs of being unhappy. They had been wrapping presents and Shannon went to her bedroom at about 10.30pm.
Mrs Doherty went to see if she wanted a cup of tea but her light was off and the key was missing from the door.
She phoned Shannon but could not get an answer and, after finding another key, she discovered the lifeless teenager. Emergency services were called but Shannon was pronounced dead a short time later.
As she left, Mrs Doherty said she placed a picture of Erin, which Shannon had kept on her bedside locker, on her chest.