NCIS Star Harmon's Bitter Family Feud!

TV LAWMAN Mark Harmon is locked in a bitter feud with his drugtroubled sister Kristin — triggered by their vicious custody battle over her son Sam more than 30 years ago!

“Mark and Kristin have tried to let bygones be bygones,” says a source close to the family. “But there’s just too much bad blood for either of them to completely forgive and forget.”

War exploded between the siblings when Mark, now 65, sought custody of Sam, following the 1985 plane crash death of his rock star dad, Ricky Nelson. Harmon claimed Kristin, now 71, was a pill-popping junkie and unfit mom.

Ricky, who divorced Kristin three years earlier, was also a druggie. He was free basing cocaine and sparked a fire, triggering the crash that killed him and six others.