Someone who was her called 999 at the time.
Two ambulance crews rushed to the address, along with an experienced paramedic and a single responder medic in a car.
A Met Police spokesman said: “Police were called by the London Ambulance Service at 12.52pm to a residential address in SW5.”
“We sent two ambulance crews, a single responder in a car and an advanced paramedic to the scene.
“Sadly, despite efforts of our medics, a person died.”
Louis is left “devastated and distraught” by the death of his sister.
Despite huge efforts to resuscitate Félicité, she was pronounced dead at the scene.
Police were immediately alerted and attended the scene to establish whether or not there were any suspicious circumstances.
Police found no signs of drugs at her home.
At the time, Louis was working in London. He was informed of her death on Wednesday night.
Last night a source close to the family said: “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.”
It comes after his mum Johannah Deakin died from leukaemia in 2016.