The 18-year-old's death is now being treated as "unexplained".
He wrote: "Such incredibly sad news today".
"Louis is obviously distraught at the loss of his sister", a source told the Sun.
Following the untimely passing of Louis Tomlinson's sister, Felicité, the "Just Hold On" singer received several messages from his friends and fans wishing him well.
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This heartbreaking news comes just two years after Louis lost his mum, Johannah, after a long battle with leukaemia. "This boy has had it tough".
X Factor winners Rak-Su also offered their prayers for the family.
It's the second tragedy to hit the Tomlinson family in recent years.
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In a statement, Scotland Yard said: "Police were called by London Ambulance Service at 12.52 on Wednesday, March 13 to a residential address. following reports of a female in cardiac arrest".
Louis' younger sibling passed away at the age of 18 after suffering a suspected heart attack.
More answers will come after a full post-mortem examination and toxicology tests are carried out. She had over one million followers on her Instagram account and was considered a social media influencer.
On January 30, she posted a picture of a row of tattoos on her forearm, which she said were the initials of all her siblings.
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Louis Tomlinson has pulled out of a planned performance at Comic Relief following his sister's death.