Lydia Wilkinson said she did not know at first whether her father Peter was going to survive the stabbing at their home in Stourbridge on 30 March.
She said looking after him was a "massive relief and an opportunity I didn't think I was going to have".
"I will miss them both so much," the 18-year-old added.
Aaron Barley, 23, of no fixed address, has been charged with their murders and with the attempted murder of Mr Wilkinson.
Lydia, who was at university at the time 50-year-old Tracey and 13-year-old Pierce were stabbed, spoke of her devastation.
Mrs Wilkinson was pronounced dead at the scene. Pierce died in hospital after paramedics battled to save him.
"For me especially it is a case of taking every day as it comes and counting your blessings and using the support around, definitely."
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She paid tribute to staff at Birmingham's Queen Elizabeth hospital who saved her father.
Lydia had been planning to collect Pierce from school the next day.
"I still can't believe that I will never see him again or pick him up from school on that Friday like I promised to do," she said.
"He was just so bubbly and full of life and he really did light up a room with his mannerisms and impressions."
Mr Wilkinson, who spent six days in intensive care, said he was "on the mend".
He suffered facial lacerations, "dozens of deep stab wounds and almost 100 stitches".
"It will take some time but obviously the emotional pain will take forever," he said.
Mr Wilkinson said: "I didn't find out until I woke up in intensive care that Pierce had not made it. It was devastating. Absolutely devastating."
He added his wife had been a "fantastic mum" and a compassionate, kind person."
"She was just beautiful, she was our angel," he said.
A student whose mother and teenage brother were stabbed to death is focusing on caring for her father who was also injured in the attack.