INSIDE the heart-wrenching family secret behind the tragic death of Martha Stewart’s beloved youngest sister.

Only 59, Laura Plimpton died of a brain aneurysm Aug. 5 in Connecticut – but years earlier, devoted Martha saved her life, a family insider exclusively told The ENQUIRER. “Laura suffered from depression and when her first husband died, she was suicidal,” disclosed the source.

“She didn’t know how she could go on living and wanted to take her own life.” Martha talked her out of it – and then gave her a reason to live.

“Martha suggested that she come to work for her,” said the source. Laura worked for her sister until she died, and became a valued colleague at Martha Stewart Living Omnimedia, writing scripts for the domestic diva’s “Ask Martha” radio show and appearing in videos on the company website.

In a 2004 interview with Larry King, Laura recalled the sad days after her husband’s death and coping with two young children. “I was just in shock. The rug was pulled out from under me,” she said, adding that her big-hearted big sister “bought me a house to live in, so that my kids could go to a nice school, and how generous is that?”

She never forgot what Martha had done. “She was always eternally grateful,” said the source.

“And she repaid Martha’s kindness, especially when Martha was incarcerated in 2004 and 2005.” Laura visited her regularly during her five months in federal prison in Alderson, W. Va. After her sister’s death, Martha, 74, posted a web tribute.

“My youngest sister, and the youngest of the six Kostyra siblings, Laura Kostyra Plimpton, passed away today,” announced the devastated media mogul.

“Laura deserves the very best; she was an extraordinary mother, wife and sister.”

Sadly, said the source, “on the day Laura died, the whole family was supposed to get together to celebrate Martha’s birthday.”