Police and NSW Health believe the teenager ingested substances before her death although a post-mortem and toxicology examination will confirm details.
Grandmother Denise Doig on Sunday said Alex was an adorable and loving teenager.
She wants Premier Gladys Berejiklian to introduce pill testing immediately so other families won't suffer like hers.
"Premier please can we have this pill testing done," Ms Doig told Network Ten.
"It's such a small thing to do. It's not hard (and) if it saves one life - one life is a life."
Five people have now died in four months after taking drugs at NSW festivals.
Uncle Phil Clark told Network Ten his niece was an only child and the entire extended family was "grieving heavily".
"Strong leadership isn't always about sticking to an ideological decision or a position," he said.
But Ms Berejiklian, speaking earlier on Sunday, insisted she remained opposed to pill testing.
"I worry that something like pill testing could actually have the opposite effect," she told reporters in Sydney.
"In the absence of evidence, we need to keep setting out the strongest message that taking these illicit drugs kills. We ask young people not to do it."
Pill testing allows people to anonymously submit samples for on-the-spot analysis to determine their composition.
A trial at a major Australian music festival in 2016 found two in three people wouldn't consume a pill if a test showed it contained methamphetamine.
Friend Evan James paid tribute to Alex on Facebook on Sunday.
"Alex Ross-King to me was the definition of a kindred spirit, she had a deeper understanding of this earth and sadly this world didn't deserve her," he posted alongside a photo of him with Alex.
FOMO organisers said they were "deeply saddened" and stressed they proactively discouraged drug use.
"Our most heartfelt and sincerest condolences go out to her family and friends," a spokeswoman said in a statement.
"Our anti-drug messaging began weeks ahead of the event and continued at the event itself."
Ten people were hospitalised after the festival which almost 12,000 attended.
Police officers searched 146 people and of those 54 had drugs.
There were 36 arrests and two people were charged with drug supply - a 23-year-old man and an 18-year-old woman.
NSW Police Assistant Commissioner Mark Jones on Sunday said it was a "very tragic event".
He said law enforcement didn't want to be "the fun police" but hoped "to make festivals safe".
Police said the drug seizures they made during FOMO were not linked to Alex's death at this stage.
NSW Health warned MDMA and other party drugs carried risks.
"MDMA affects everyone differently but its lethal toxicity is well known," the agency said in a statement.
"People should be aware of poisoning symptoms - a fast heartbeat, high body temperature, confusion and vomiting and get to medical help fast."