Leslie Carter's Family "Completely Blindsided" by Her Death

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Gone far too soon.

Aaron and Nick Carter and their entire family are reeling from news of sister Leslie Carter's shocking Tuesday death.

At just 25 years old, the aspiring singer, wife and mom to daughter Alyssa, 10 months, passed away of unknown causes in upstate New York.

"The family legitimately doesn't know what happened," a source close to the Carter family tells Us Weekly. "They're still trying to find out."

Singer Aaron, 24, is "completely blindsided" by the news. Leslie, whose single "Like Wow!" was featured on the Shrek soundtrack in 2001, had four siblings and married Alyssa's dad, Mike, in 2008

"She loved her family, she loved her baby," the source adds of Leslie, whose 1999 debut album was never released. "She was happy stepping away from the show business and just living happily with her family in Canada."