James Gallagher said 15-year-old Shannon Gallagher was "missing her wee sister" as she prepared for Christmas without her sibling, Erin, 13, who committed suicide two months ago in Donegal.
Shannon, also from the Irish town, is believed to have killed herself after being found dead on Wednesday night.
Mr Gallagher said: "Erin died on October 27 and Shannon's been grieving really hard and she's been all over the place.
"I spoke to her the other night and she was making plans about coming to Scotland to talk to me and to stay with me. She was going to go to university here, but it never happened.
"She had good days and bad days and was up and down.
"I just know that she was on Facebook last night talking to my son and she said she was finding it quite hard, she was wrapping her presents and she was missing her wee sister."
He added: "She was wrapping the presents up for her mum and wee brother and people, then she writes on Facebook saying 'this is the hardest thing I've ever had to do in my life baby doll'.
"That's what she called her wee sister: baby doll."
Erin is believed to have been the victim of cyber bullying prior to her death. The teenager was found dead after telling friends on a social networking site that she was considering killing herself after a campaign of abuse.