His sister Felicite - who was only 18 years old - passed away on Wednesday afternoon after an ambulance was called to her flat in West London. Currently, The Sun says the young woman's death is being classified as "unexplained" by authorities, pending a full autopsy and toxicology test.
He tweeted: "Such incredibly sad news today".
His mother Joannah died in 2016 after a battle with leukemia.
Watford boyband Rak-Su, who won the fourteenth series of the X Factor before Tomlinson debuted as a judge, sent their sympathies.
Despite attempts to resuscitate Tomlinson, she was pronounced dead on the scene, TMZ said.
Vietnam Asks Malaysia to Free Woman Accused of Poisoning Kim Jong Nam
No reason was given. "I am sure they decided that it was an easy thing to release her and score a few points with Indonesia ". Defense lawyers have maintained that the women were pawns in an assassination orchestrated by North Korean agents.
Two ambulance crews reportedly responded to a 999 call (London's emergency call number), as well as a paramedic and medic in a separate vehicle, The Sun said.
Louis' six siblings (from left) Felicite, Lottie, Phoebe, Daisy and Ernest.
It added: "A post-mortem examination will take place in due course".
Felicite, who was also known as Fizzy, was an aspiring fashion designer and Instagram influencer with 1.3 million followers.
He had been scheduled to perform his new single Two Of Us, which was written in honour of his late mother, on Comic Relief on Friday (March 15), but has now cancelled the appearance.