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.

Darren Heath, 45, from Taynton, was sentenced to three years and nine months after pleading guilty to five counts of misconduct in public office.
Bristol Crown Court had heard how Heath got one of his victims pregnant and convinced her to have an abortion.
Gloucestershire Police believe there may be more victims.
In sentencing, Judge Neil Ford QC described Heath's behaviour as "corrosive to the reputation of the police service generally".
The force said Heath used his position to identify women - "often victims of crime who were already extremely vulnerable" - whom he could offend against.
He was described as "manipulative and calculating" by the force, which suspended him last year following an investigation into a complaint made against him in 2012.
The offences took place between 2002 and 2012.
Previously, Bristol Crown Court had heard that Heath was first warned about his behaviour in 1996 - a year after joining Gloucestershire Police - following a complaint by a student.
He had asked her if she "fancied a bit of fun".
"She subsequently made a complaint which was dealt with by Pc Heath being given advice."
Heath met his first victim in 2002 after she was arrested for drinking and driving.
Six years later he met another victim who approached him in a police car after a man fell unconscious in the street.
He later visited her home where the pair had sex and then persisted with visits - up to four times a week - before the woman fell pregnant with twins.
Heath, who has two children of his own, then convinced her to have a termination.
He met another victim after arranging a restorative justice programme for her son.
The woman ended the relationship after realising Heath was "only interested in a sexual relationship", the court heard.
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.
[NEXT_CONCEPT]
A "manipulative and calculating" police officer who abused his position to have sex with vulnerable women he met while on duty has been jailed.