She had undergone surgery two days earlier, on the Wednesday, to have her tonsils and adenoids out and had been discharged the following day.
Miss Allen-Janssen, who lives with her mum and Danielle, said her sister went to bed "perfectly fine" on Thursday night.
She was also fine when her mum checked on her in the middle of the night, but when they went to check on her Friday morning she had died.
The family are still waiting on some test results, but for the time being are focused on farewelling Danielle today.
"She was so excited for Christmas. She loved Christmas."
Miss Allen-Janssen said the family sat next to Danielle's casket opening her presents for her.
"She had heaps of presents and she would have loved every single one of them, especially the pool."
The new togs and boogie board would also have been a hit with the water loving kid.
She described her sister, who was born on her 18th birthday, as "animal obsessed", who wanted to be a vet when she grew up.
"She used to pretend she was a cat or a dog. She was a really unique child . . .
"She used to go into the garden and pick up worms and snails to put them in an insect container."
Pink was her favourite colour, and while she was a "girly girl" she loved getting out and doing things, Miss Allen-Janssen said
"She was a very adventurous girl."
Danielle loved to sing and dance - one of her favourite songs was Katy Perry's Firework - and she enjoyed being the centre of attention.
"Just before Christmas we were at the supermarket checkout and she started singing and dancing to [the song] at the checkout. Everyone was staring at her smiling."
Miss Allen-Janssen said it was a special memory she would hold on to.
"She loved colour, dance and music. She was just a really bubbly bright child who had a lot of friends. Everyone needs to know how special she was."
Miss Allen-Janssen said her sister had a real love of cars which she'd inherited from her dad - and was able to name the makes and models of many cars.
Danielle will be taken to her funeral service today on the back of her dad's ute, travelling past the school she loved one last time before being met at the crematorium by kids with sparklers. The family also plan to release 99 helium balloons.
Miss Allen-Janssen said her baby sister was around the same age as her four nieces and nephews who ranged from 2 to 10 years old, with some of the children struggling to understand what had happened.
"Everyone is holding up as best as they can. It's a horrific time for everyone and a time that should have been so happy."
To help ease the pressure with the unexpected funeral costs, extended family have set up a Givealittle page. To donate go to https://givealittle.co.nz/cause/daniellejoysfuneralcosts
Meanwhile, Lakes District Health Board communications officer Sue Wilkie said the board extended its "deepest sympathy" to Danielle's family.
She said the board could not comment on the case because it was a coroner's investigation.
"Once a case review has been carried out, that information will be passed to the coroner for scrutiny as part of his coronial investigation. Work has begun on preparing for the case review, which is expected to occur during January."
