Louis Tomlinson's sister, Felicite, died after suffering a suspected cardiac arrest at the age of 18.
She died at her apartment in Earl's Court, west of London, Wednesday afternoon.
Emergency services attended the residence following reports of a woman in cardiac arrest.
In a statement, the London ambulance service said: "We were called at 12.51 pm.
"We sent two members of the crew, one rescuer in a car and an advanced paramedic on the scene.
"Unfortunately, despite the efforts of our doctors, one person is dead."
The police said the officers arrived shortly after the ambulance teams, but the female was declared dead on the spot.
At this stage, Felicite's death is treated as "inexplicable" and a post-mortem will take place in due course.
According to The Sun, the singer One Direction has been left "devastated and upset" by the death of his sister, who has been described as "lovable, caring, bright and beautiful".
Felicite, which has 1.3 million followers on Instagram, was an aspiring fashion designer.
The 27-year-old singer and judge of X Factor was in London for an appearance now canceled in the Comic Relief on Friday night when he was informed of the death on Wednesday night.
The police also notified other family members from Doncaster, South Yorks.
Tomlinson's mother, Johannah Deakin, died of leukemia in 2016 at the age of 43 years.