The 18-year-old died from a suspected heart attack on Wednesday, March 13.
The terrible news comes after the singer lost their mother, Johannah, from leukemia back in 2016.
A family source told the UK's Sun newspaper the singer was left “devastated and distraught” by the death of his sister.
The source said: “Felicite 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 lovable, caring, bright, passionate, popular and beautiful young lady.”
The aspiring fashion designer and social media influencer collapsed after a suspected heart attack at her fourth-floor studio apartment in Earls Court, West London.
Two ambulance crews dashed to the home, along with an experienced paramedic, however she could not be revived.
Police attended the home as a precaution to establish whether there were any suspicious circumstances.
The death of Felicite — whose friends called her "Fizzy" — is being treated as “unexplained” at this stage.
A full post-mortem examination will take place and toxicology tests will be carried-out to determine the full cause of death.
Devastated Louis is being comforted by his family and friends in the UK as they prepare for his sister’s funeral.
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