FULLERTON, Sept. 17 — Russell Harmon searched for 20 years for his younger adopted sister, Rosemary, who dropped out of sight during the 1950s.

Not until two years ago did Harmon discover that his missing sister was Rosemary LaBianca, one of the six persons who were brutally slain by the Charles Manson family during two nights of terror in August 1969.

Now Harmon, 53, is searching for Rosemary’s two children, Susan and Frank Struthers, so that he can give them some mementos of her younger life.

“I’d like to give them some of the personal effects of their mother’s, like her adoption papers, pictures, high school diploma,” said Harmon, an assistant engineer with Ford Aerospace and Communications Corp. in Newport Beach.

Harmon said he discovered the slain Rosemary LaBianca was his sister by accident shortly before Christmas, 1974.

At the time of the Manson slayings, he said, the Harmons had gone through a series of personal tragedies including the deaths of a grandson, a son who died in Vietnam and his mother.

“So, whenever anything came on television about the Manson murders or the trial, I would say ‘turn it off’ because I just couldn’t take it.

“We even wouldn’t read the stories about the murders in the newspapers. It was just too much.”

They finally learned about Rosemary’s death when Vincent Bugliosi, who prosecuted the Manson clan, published his book about the murders, “Helter Skelter.”

“My cousin called me and told me to get Bugliosi’s book and read such and such a page,” Harmon’s wife, Norma, said. “‘I know it’s Rosemary,’ my cousin said.

“I went right to the book store, bought the book and when I read it I was so stunned I could hardly drive home.

“When I got home I told Russ I had a terrible Christmas present for him. When he heard the news he was just shocked. He tried all afternoon to get in touch with the district attorney’s office to get a hold of the kids, but he had no luck.

“It was the worst Christmas ever.”

Harmon said his parents adopted Rosemary from an Arizona orphanage when she was 8 years old and “we were as close as a sister and brother could be.

“In fact, the night before I went into the Army during World War II, I took my sister out for a celebration dinner instead of my girl friend.”

Harmon said his sister moved away soon after graduation from high school and married her first husband, Frank Struthers, by whom she had two children, Susan and Frank Jr.

It was while she was divorcing Struthers that they lost contact, he said.

“We tried and tried to find her through the years but couldn’t. I think I checked every telephone book in Los Angeles looking for Rosemary Struthers. I didn’t know then that she had divorced and remarried.”