In an ENQUIRER World Exclusive, Angelina Jolie’s biographer ANDREW MORTON goes behind the scenes to reveal the untold story about her brave decision. The English journalist and writer, who also penned best-selling biographies on Princess Diana and Tom Cruise, published his book “Angelina: An Unauthorized Biography” for St. Martin’s Press in 2010. Here’s his report:

BRAVE actress Angelina Jolie’s dramatic decision to undergo a double mas­tectomy was sparked by her beloved aunt’s battle with breast cancer!

Sources tell The ENQUIRER that Angelina secretly agonized over the decision for nearly 13 years, and finally chose to go under the knife after the younger sister of her moth­er Marcheline Bertrand– who succumbed to ovarian cancer at age 56 – was stricken with breast cancer when it returned for a second time.

The Oscar-winning actress and mother of six, 37, hesitated to have the drastic surgery, even though her mother, grandmother and great-grandmother all died of cancer at a young age, insiders say.

But when Angelina learned that her aunt Debra Martin, a 61-year-old mother of two from Escondido, Calif., was battling stage 4 breast cancer, she decided to go ahead with the operation, sources say.

Speaking publicly for the first time about Angie’s life-changing decision, Debra said: “I believe this was the right thing to do. I think it has reduced her chances of getting breast cancer dramatically. If I had informed myself better before I got breast cancer, I would have made the same decision.”

Debra had tried to beat the family curse after Angie’s mother was diagnosed with ovarian can­cer in 2000. Like Marcheline, Debra tested positive for the faulty BRCA1 gene, a marker for increased risk of both breast and ovarian cancer, and decided to have her ovaries removed.

Despite that, she was diagnosed with breast cancer in 2004. Sur­geons performed a lumpectomy, and Debra seemed to have beaten the Big C.

BUT last year the disease returned with a vengeance. By Janu­ary, Debra had wasted away to just 96 pounds.

Her condition then stabilized, but her husband Ron spent hours on the phone each day trying to find a clinical trial that would accept her and provide cutting-edge treatment to beat the body-ravaging scourge.

Sadly, Debbie Martin lost her battle with breast cancer on May 26. She was 61.

It was Debra’s gutsy decision to undergo the preventative ovarian surgery that inspired Angelina, said her mother’s longtime partner Bill Day, who lived with Marcheline for 11 years.

“Debra is the unsung heroine of this story because she was the first woman in the Bertrand family to attempt preventative surgery,” said documentary filmmaker Day. “She has been a shining example to Angelina.”

FOR THE FULL STORY with all the details about how the dread disease is killing Angie‘s family members one by one – it’s ONLY in the new ENQUIRER!