A Soviet Pentecostalist today won an unprecedented concession from authorities who permitted her to return to the U.S. Embassy for a tearful reunion with her family before flying home to Siberia Friday.

Lydia Vashchenko, 30, was allowed to visit the embassy compound 12 days after American officials rushed her to a Soviet hospital for treatment following her 36-day fast to dramatize the family's demand to emigrate to the United States.

She was one of seven Pentecostalists, a fundamentalist Christian sect here, who raced past Soviet police guards into the embassy on June 27, 1978, saying they were subjected to religious persecution. They have lived in a single basement room inside the compound ever since.

American officials said they were pleased with today's development, which raised the possibility that the authorities may grant exit visas to the Pentecostalists if they comply with Soviet regulations and make formal applications in their home town.

Since Vashchenko and her mother, Augustina, 52, began their hunger strike Dec. 28, a number of Protestant groups and prominent individuals in the United States and other countries have appealed to Soviet authorities to allow them to leave.

There was considerable concern that the two women would need urgent medical attention and several prominent figures, including former president Jimmy Carter, had spoken to them by telephone to urge them to abandon the protest. Lydia Vashchenko was rushed to Moscow's Botikin Hospital after the embassy doctor decided her life was in danger.

Vashchenko subsequently gave up her fast. She appeared today to be in good health.

There has been no formal Soviet statement on the issue, and American officials said they did not know until the last moment that Vashchenko would be allowed to enter the U.S. compound.

Security around the U.S. mission here is exceptionally tight, and several persons who recently entered the compound without permission were arrested almost immediately after leaving.

Today, however, the guards stood aside and waved in a U.S. diplomatic car carrying Lydia Vashchenko and her brother Alexander and sister Vera, who previously had not been inside the embassy.

During a joyous but tearful reunion, burly 22-year-old Alexander, who had not seen his parents and sisters for more than three years, lifted his frail father Pyotr off the ground in a bear hug. Vera, 25, sought to hug them all at one time.

Augustina Vashchenko said she was "very grateful" for the family reunion and voiced thanks "to all people in the West" who supported their attempt to emigrate.