Singer Louis Tomlinson, a star of the popular boy band One Direction, is mourning the sudden loss of his younger sister, Félicité Tomlinson.

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The model and social media star, 18, collapsed Wednesday at her London apartment from a suspected heart attack, according to news reports.

The Metropolitan Police Service told E! News that when emergency responders arrived, they found a female in cardiac arrest who was pronounced dead at the scene.

"Police are in the process of informing her next of kin," E! reported, according to a police statement. "At this stage the death is being treated as unexplained. A post-mortem examination will take place in due course."

The loss of Félicité Tomlinson comes just two years after the family lost their mother, who died from leukemia.

Louis Tomlinson, 27, just released an emotional tribute song to his mother called “Two of Us,” who died in 2016.

The last post Félicité Tomlinson shared on Instragram is a selfie of her sitting on the floor with the caption, “Don’t know why I look so shocked,” with a heart emoji.