As a child, it was sometimes difficult to understand what was going on and he admitted initially blaming himself for Phoebe's illness.
Grant Hawthorne.
"For a while, I thought that it was my fault that Phoebe had cancer. It's just when you don't understand, you just think: 'My gosh, my little sister's really sick - did I have something to do with it'?
"When you're that age, or any age, no one ever expects that will happen to them and it was really scary. When you're told about cancer, as a kid, you only hear about it when it kills people - and that's what we thought. But mum and dad quickly pointed out that that's not necessarily what's going to happen."
For three years, Phoebe continued with cancer-fighting treatments and the family, from Wellington, rallied to work around her hospital appointments.
When she was 5 years old she went into remission and the Hawthornes were told their little girl was officially cleared of cancer.
Mr Hawthorne, who is studying geology at the University of Auckland, has been involved with charity CanTeen since he was 13.
Today the charity holds its annual fundraising appeal, this year dubbed the CanTeen Never Alone Appeal - encouraging the idea that young people with cancer should never feel alone.
Mr Hawthorne said his involvement with the charity had let him help other young Kiwis going through what his family went through.
"That's what's good about CanTeen - everybody's got that understanding. Everyone's story is different, but you know what it's been like. We've had similar experiences and we know how to look out for each other."
He said he enjoyed being able to help other young people. "I can't believe [Phoebe's] 12 now - she's just a blessing and the light of the family. She's just a cool little kid."
• To donate, visit www.neveralone.co.nz or www.canteen.org.nz