The Soviet government altered telegrams and released year-old videotapes of Andrei Sakharov to hide the effects of the dissident physicist's six-month hunger strike this year, according to close family members here to greet Sakharov's ailing wife, Yelena Bonner.

The relatives said the moves were intended to prove that Sakharov was off his hunger strike and also to isolate the Sakharovs more fully from the West and their friends and to disguise the reality of the couple's life in exile in the Soviet city of Gorki.

"All this shows us plainly what mother said years ago," Bonner's son, Alexei Semyonov, said. " 'We could be killed in Gorki and no one would know.' That has become a frightening reality."

Sakharov, the winner of the 1975 Nobel Peace Prize, has become the leading symbol for westerners protesting Soviet human rights violations, and his case has been made a high-priority issue by the Reagan administration in its dealings with the Soviet Union.

Since his direct contacts with the West and even with friends in the Soviet Union were largely cut off six years ago, only sporadic and unverifiable information about Sakharov has filtered to the outside. Today's account by family members after talking to Bonner, who arrived here from Moscow last night, indicated, however, that while he has been in poor health, after hunger strikes and a 1984 stroke, he has recovered sufficiently to resume exercising and doing scientific work.

Sakharov was one of the Soviet Union's most prominent nuclear physicists until he became an advocate of improved human rights and greater democracy in his country.

He was banished in 1980 to internal exile in Gorki, a city 250 miles east of Moscow that is off-limits to foreigners. Bonner joined him there in 1984 after being sentenced to five years of internal exile on charges of slandering the Soviet state.

Today Semyonov and Bonner's son-in-law, Yefrem Yankelevich, who live in the United States, held a press conference here to discuss Sakharov and Bonner a day after she arrived for treatment for an eye ailment at a Siena clinic where she has been treated three times before.

Soviet authorities gave her permission to come here, and later to go to the United States for heart treatment, only on the condition that she not talk or otherwise communicate with the press.

In meeting with reporters today, both Yankelevich and Semyonov emphasized that their comments were based not on Bonner's statements since she arrived here but on telephone calls they had with her and Sakharov in Gorki last month after Soviet authorities granted Bonner permission to visit the West.

Bonner, Semyonov said, "signed an agreement" not to communicate with the press in order to be allowed to seek medical treatment and then to return to her husband, "and she intends to keep her word."

Yankelevich said he did not consider his and Semyonov's remarks today "a breach in the agreement she made. We are not under any obligation to the KGB or CIA or anyone else."

Semyonov, who insisted that, since her arrival here, his mother had only "corrected" some of the information they previously received from Moscow, charged that during Sakharov's six-month hunger strike this year to gain permission for his wife to get medical treatment, virtually every domestic telegram the couple sent to friends in Moscow was altered by authorities to conceal the extent of the hunger strike.

"None of the telegrams my mother and Sakharov sent to friends during this period reached them in the form they were sent," Semyonov said. He said this was done even though most Soviet telegrams are facsimiles of written messages.

One example, according to Semyonov, was a cryptic telegram Bonner sent to friends quoting a line from a well-known Soviet poem: "I have friends, thank God." The line had been written, he said, in the expectation that the friends would remember the line that precedes it: "The solitude is driving me from door to door." He said Bonner had hoped the line would communicate the fact that she and Sakharov were separated in Gorki, with Sakharov in a hospital, where he was being force-fed by authorities.

But when the telegram reached their friends it had been changed "with words from previous telegrams," Semyonov said, to read simply, "Everything is all right, thank God." Both Bonner's and Sakharov's signatures were on the cable, even though on the original, only Bonner had signed, he said.

The videotape of Sakharov in the hospital, walking around and eating normal meals, that was widely shown in West Germany last summer -- apparently released by Soviet sources to disprove reports he was suffering from a prolonged hunger strike -- was equally misleading, Semyonov said. He said the tape was at least a year old.

At the time it was being shown, he said, Sakharov had renewed his hunger strike after a two-week pause to find whether the new Soviet leader, Mikhail Gorbachev, would be more responsive to his pleas than was his predecessor. When he was not, Sakharov resumed his hunger strike until Oct. 23, when Bonner was granted permission to go abroad, Semyonov said.

Sakharov's health, he said, remained "precarious" after his long hunger strike, although he had not suffered a stroke, as he had while being force-fed during a 1984 hunger strike. "Nevertheless," Semyonov said, Sakharov "says he feels good, is doing exercise every day and has started his scientific work again."