"It's been a harrowing couple of days so the shock is slowly turning into more full-on grief."
Mr Gillman-Harris travelled from Auckland to Wellington to spend Christmas with Ms Harris and her family.
"Like everyone else, he was full of Christmas spirit," she said.
"It was a special time, even more so now."
On Boxing Day, he left her home to make the journey back north.
The next day he was dead.
A police investigation was launched after Mr Gillman-Harris sustained a fatal head wound during a stay at the Ascot Epsom Motel.
CCTV footage was released showing two youths he allegedly checked in with - West Auckland teens Beauen Daniel George Wallace-Loretz and Leonard Nattrass-Berquist.
Police would not be drawn on the circumstances of that meeting and Ms Harris was similarly steadfast.
"First and foremost, his life was taken," she said..
"We're really keen to let the justice system take its course and any noise that's possibly coming out around things like that, all the more reason why our priority is holding fast on looking after our whanau to get through this."
Ms Harris was born and bred with her brother - along with five other brothers and five sisters - in Tokoroa.
He was one of the founding pupils at Forest View High School and his sister said he was "very bright".
Mr Gillman-Harris spent time in Europe for work and a love of languages meant he learned German, Swedish, Dutch and Te Reo Maori.
He worked for KiwiRail as a team leader assistant in Auckland's Southdown freight terminal and many of his colleagues made the drive north for his funeral.
"As many Kiwis welcomed in the New Year, we farewelled a much-loved brother, uncle, workmate and friend in a manner that was in stark contrast to how his life was so brutally taken," Ms Harris said.
"I'll remember his happy, smiley, cheeky face. I'll remember him as a brother who would do anything for his whanau and who loved them all very much."
A post-mortem examination revealed Mr Gillman-Harris died as a result of blunt-force trauma to the head.
The pair accused of killing him will appear in the High Court at Auckland next month where they will enter pleas.
They are expected to apply for bail before then.