MARTHA STEWART is  grieving the loss of younger sister Laura Plimpton, who died at 59 following a brain aneurysm.

Stewart, addressing staffers, wrote in an e-mail, “My youngest sister, and the youngest of the six Kostyra siblings, Laura Kostyra Plimpton, passed away today in Norwalk Hospital. 

“Laura worked for me and the company for more than 25 years”

Martha then documented the trail of events leading to the premature death — how Plimpton complained about feeling ill after a workout, at which point her husband Randy took her to the hospital for immediate care.

“We are so sad,” she wrote, “but also happy that Laura had the foresight to donate so many healthy organs to needy people.”

The domestic diva called her beloved sibling, who had three children, “an extraordinary employee, mother, wife and sister” in the remembrance.

Plimpton, who’s first husband died of cancer, had a “very special relationship” with Stewart, appared on the Larry King Show a decade ago. She told the talk show host that Martha was like a mother to her.

“All I wanted to do was be around her,” Plimpton had said. Condolences…