"She had a seizure and we took her into the hospital," her husband Ronald told UK newspaper The Mirror.
"She's in a coma. It's breast cancer but I think it spread everywhere.
"They have turned off the life support and her family is all here holding her hand," he said.
A source told the outlet Jolie was "very close" to Martin.
Relatives reportedly call her the "unsung" heroine in Jolie's quest to admonish the disease from her life.
Martin recently revealed she pushed Jolie to take action and have the mastectomy and afterwards applauded her niece for being so proactive about her health.
Martin underwent a lumpectomy following her stage four breast cancer diagnosis in 2004, but that didn't stop the disease from returning stronger than ever in 2012.
"I believe this was the right thing to do," Martin told the National Enquirer of Jolie's surgery. "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."
Tests have showed Jolie is also at an increased risk of developing ovarian cancer.
The actress is believed to be planning to have her ovaries and uterus removed before her 40th birthday to further lower her risk.
Jolie's mother, Marcheline Bertrand, died from ovarian cancer in 2007. She was 56.
The actress touched on her family's heart-breaking struggle with cancer in the Times article.
"My chances of developing breast cancer have dropped from 87 per cent to under five per cent as a result of the February procedure," Jolie wrote.
"I can tell my children they don't need to fear they will lose me to breast cancer."
Angelina shares six children with fiancée Brad Pitt.
- AAP
