CAROLINE KENNEDY is facing yet another family tragedy – she’s losing her beloved aunt LEE RADZIWILL to Alzheimer’s disease.

And insiders say the 79-year-old sister of former first lady Jackie Kennedy is so ravaged by the heartbreaking illness that she believes JFK is still president!

“It’s terribly sad,” shared a family friend. “Lee was always so vital and engaging. But now she seems trapped in a 1960s time warp.”

Caroline, 54, whose life has been filled with loss, has been spending hours at Lee’s bedside with her cousin Anna Christina, trying to keep Lee comfortable.

“It’s a tragic situation,” continued the friend. “It’s especially difficult when Lee says to her, ‘Caroline, darling, how’s your mother? Let’s go see Jackie and Jack at the White House!’”

The timing is especially difficult for Caroline, as the 18th anniversary of her own mother’s death, May 19, is fast approaching.

“It breaks my heart to see my aunt so mentally frail and confused,” Caroline confided to a pal, according to the source. “She deserved a better finale for a life well-loved and well-lived. After everything my family’s been through, I just wish Ic ould have a few years free of death and tears.”

Lee, a former jet-setter and socialite, was married three times – her second husband was Polish prince Stanislaus Radziwill. The couple had two children, Anna Christina  and Antony, who died at age 40 in 1999 from cancer, just three weeks after the death of his cousin and best friend, JFK Jr.

The only upside to the situation is that Lee isn’t able to remember the tragedies that have plagued her family over the years.

Added the friend: “All she remembers is the love, laughter and excitement during the days of America’s Camelot.”