Louis Tomlinson’s sister Félicité died Wednesday in London at 18.
According to reports, paramedics responded to a call of a female in cardiac arrest at 12:51 p.m. local time but were unable to resuscitate her. Félicité was pronounced dead at the scene.
Her death is being treated as "unexplained."
The Sun reported that the teen, an aspiring fashion designer and social media influencer known as "Fizzy", collapsed in her fourth-floor apartment and an unidentified person who was with her called 999 (the British emergency number).
Tomlinson lost his mother Johannah Deakin to leukemia in 2016 at the age of 43.
The former One Direction singer was in London at the time of his sister's death preparing to perform his new single "Two of Us" at Friday night's Comic Relief telethon on the BBC.
"Two Of Us" was inspired by the loss of their mother.
“You’ll never know how much I miss you / the day that they took you / I wish it was me instead," he sings. “I will be the best of me / Always keep you next to me / I’ll be living one life for the two of us.”