The One Direction hitmaker's sibling died at her flat in Earl's Court, west London, and her death is being treated as unexplained by police.
Louis Tomlinson has pulled out of a planned performance at Comic Relief following his sister's death.
For the singer, his sister's death comes just two years after the tragic loss of his mother, Johannah Deakin, who lost her battle with leukemia at age 42 in December 2016.
Singer Charlie Jones tweeted that he "can't imagine the pain" Tomlinson and his family are going through.
Meanwhile, Rak-Su, who also appeared on The X Factor, added: "Our condolences and prayers go out the @Louis_Tomlinson and his family at this time".
A spokesperson for the Met said: "Police were called by the London Ambulance Service at 12.52pm to a residential address in SW5". Paramedics couldn't revive her, and she was pronounced dead at the scene.
A person who was with her called 999, according to the Sun newspaper.
Félicité, known to friends as Fizzy, was a social media influencer with more than one million followers online.
Both Felicite and Louis were close as the One Direction member spent time with his sister whenever he was in London, TMZ reported.
It's the second tragedy to hit the Tomlinson family in recent years.
He has canceled his scheduled appearance on UK TV for the yearly charity event Comic Relief.
Louis released a song last week which was inspired by the grief he felt over his mum's death.