The model and social media influencer reportedly collapsed on Wednesday after a suspected heart attack at her London apartment.
Louis Tomlinson back in 2014 taking selfies with fans.
Watford boyband Rak-Su, who won the fourteenth series of the X Factor before Tomlinson debuted as a judge, sent their sympathies.
Singer Charlie Jones tweeted that he "can't imagine the pain" Tomlinson and his family are going through.
An ambulance was called, but paramedics were unable to revive her and she was pronounced dead at the scene.
A Metropolitan Police spokesman said: "Police were called by London Ambulance Service at 12.52pm on Wednesday, March 13 to a residential address in SW5 following reports of a female in cardiac arrest".
More answers will come after a full post-mortem examination and toxicology tests are carried out. Louis is yet to react publicly to the tragic news but according to British newspaper The Sun the singer is "devastated and distraught" by the loss of his sibling who was "lovable, caring, bright and beautiful".
The late star was well-known on social media and regularly shared posts with her 1.3 million followers on Instagram.
He has canceled his scheduled appearance on UK TV for the yearly charity event Comic Relief.
Felicité - who was also known as "Fizzy", to her friends - passed away on 14 March, and her death is being considered as "unexplained" at this stage.
The siblings' mother died at the Royal Hallamshire Hospital in Sheffield on December 7 2016 after being diagnosed with leukaemia earlier that year.