"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 freebasing cocaine and sparked a fire, triggering the crash that killed him and six others.
In 1987, after Kristin did a stint in rehab for an addiction to prescription painkillers, Mark — who plays Jethro Gibbs on CBS' hit NCIS — and actress-wife Pam Dawber went to court seeking permanent custody of Sam, then 12.
"Mark and Pam claimed Kristin wasn't a good mother because of her drug dependency at the time," says a source.
On the stand, Dawber, now 65, testified she'd seen Kristin "blasted" while shopping and said Sam was afraid to go home because his mother acted "erratic."
Meanwhile, Kristin's lawyers accused Pam of using cocaine while working on ABC's Mork & Mindy, co-starring the late Robin Williams, during the sitcom's 1978 to 1982 run.
Shortly before Pam was to testify about her drug use, which she denied, Harmon suddenly dropped the suit.
Kristin's lawyers branded it an "unconditional surrender."
A musician and actor, Sam, now 42, lives in Sherman Oaks, Calif. Kristin became an artist in Santa Fe, N.M.
She says Mark dropped the case "rather than prolong the agony for everyone in the family."
But Kristin sadly adds: "I knew from that time on that our family would never be the same again, and it hasn't been."
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