Jodie Chesney's dad has said that her sister is too scared to leave the house after the teenager was stabbed.
The 17-year-old died after she was knifed in the back in a park in Romford, east London, on March 1 this year after her killer mistook her for someone else.
Speaking on Good Morning Britain, Peter Chesney told how his other daughter Lucy, 20, has been completely changed by the murder.
Peter, 39, said: "I've got another daughter Lucy and I have to, I want to, be strong for her. She's lost her best friend.
"She's changed a hell of a lot. We're all just dealing with it at one day at a time."
Peter added: "She's changed so much. She's frightened to go out."
The dad said he was having to take it all "one day at a time" as he grieves for his daughter.
He added: "I've said before you have to do that one day at a time. You're not trained for this, no human being is trained for this level of grief so you have to deal with it."
On Thursday Scenson Ong-a-Kwie, a drug dealer, and a 17-year-old boy, who cannot be named for legal reasons, were convicted of Jodie's murder at the Old Bailey.
Jurors were told that Ong-a-Kwie, who was the knife man, stabbed Jodie in the back mistakenly thinking she was a rival gang member.
Peter said: "It was a mistake and it is quite incompressible, how do you come to terms with something so senseless when she did nothing wrong, literally nothing wrong?
"I will never come to terms with it and neither will my family."
He discovered his daughter had died when he overheard it on a police radio while he was being rushed to the scene of the attack.
Peter continued: "It was all so bizarre and crazy, it was like riding a rollercoaster. I did remember the radio announcement saying 'Take Jodie's dad home because she's gone.'
"I just dropped to the floor in the police van."
He has now set up the Jodie Chesney Foundation that works to prevent knife crime.
Peter said: "Young people nowadays are so much more willing to use knives and hurt people in a ferocious in a deadly manner.
"I won't sit here and pretend to know what's going on with young people and the culture but I think it's a cultural thing, we need to get into the deep root into why young people want to hurt other people."
