Rebecca Harden, 37, succumbed to her injuries at Mount Sinai Hospital of Queens, a hospital spokeswoman said yesterday.
Her son, Sander Waring Harden, died Sunday after fire erupted in the Astoria apartment when a lit candle ignited a sofa. Audrey Gay Harden died two days later.
Private funeral services for the children and their mother will be held Saturday in Manhattan.
Rebecca Harden was divorced from the youngsters' father, Thaddeus Harden, who is the Academy Award-winning star's brother.
"The family is in mourning," Carri McClure, the actress' spokeswoman, said yesterday, telling a reporter that they are "thankful for the tremendous level of support they have received, and we hope that you appreciate and respect the family's need for privacy at this difficult time."
Virginia Braster, an East Side neighbor of Rebecca and Thaddeus before the couple split up, said her heart went out to the "wonderful" dad who doted on his kids.
"I can't imagine what it's like to go through this," she said, noting that the two children were their mother's "heart and soul."
"They were really blessed children," she said. "You couldn't help falling in love with their personalities. Audrey at 3 could tell you anything you wanted to know about everything."
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