According to reports, Félicité Tomlinson died in her apartment in London. An unidentified individual, who was with Félicité at the time of the death, called first responders after she collapsed due to a suspected heart attack.
Despite efforts by medical personnel to resuscitate her, she was pronounced dead in her apartment. A full post-mortem examination and toxicology tests will take place to determine the exact cause of the the Instagram influencer's death, the Sun reported.
According to a family source who spoke to the Sun, "Félicité was an absolutely adored young woman who was loved by Louis and her whole family."
"They are all totally devastated, as you can imagine. She had so many friends and was such a positive happy person. It's a massive loss to the world. She was a loveable, caring, bright, passionate, popular and beautiful young lady," the source added.
Following the news of his sister, the English pop star pulled out of a Friday performance on BBC's "Comic Relief."
Félicité owned a clothing line, called Fizzy, and had 1.4 million followers on Instagram. She was an aspiring fashion designer and author. In October, she revealed on Instagram that she had been suffering from sciatica, a back pain condition caused by issues with the sciatic nerve, which runs from the lower back down each leg.
Félicité's death comes three years after her mother, Johannah Deakin, died of leukemia in December 2016 at the age of 43.
One week ago, Louis released a new song dedicated to his mother called "Two of Us," in which he sings directly to her.
"You'll never know how much I miss you/The day that they took you, I wish it was me instead," Louis sings. "But you once told me don't give up/You can do it day by day/And diamonds they don't turn to dust or fade away."