In a new interview with Modern Luxury Manhattan, Chastain says her sister Juliet Chastain "had struggled a long time with drug abuse."
"And she had a lot of attempted suicides, but you never really think this is going to happen, even though, in your mind, you know it can happen," the The Huntsman: Winter's War actress told Modern Luxury. "And when you get the call, it's ... shocking."
Chastain was just three days away from her Julliard School graduation in 2003 when the then-24-year-old took her own life. Juliet was Chastain's only full biological sibling: she has a half-brother, Will Chastain, from her mother, and two estranged half-sisters, Nicole Monasterio and Laci Smoot, on her late father's side.
Juliet's shocking death, as well as those of the actress' Othello costar Phillip Seymour Hoffman and of Robin Williams – who provided Chastain's Julliard scholarship – in 2014, inspired her to get involved with nonprofit To Write Love on Her Arms, she told Modern Luxury.
To Write Love on Her Arms is dedicated to providing support for those struggling with depression, addiction, self-injury and suicidal thoughts.
Chastain previously told InStyle in 2015 that Juliet's suicide "completely changed the person I am."
"A movie, Oscars, a dress, if someone thinks I'm stupid … I realized nothing is that important."
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The star also noted to Modern Luxury that she is "an emotional person," adding, "If someone gets my heart, then they kind of get me forever."