Soviet authorities have agreed to allow the step-daughter of dissident physicist Andrei Sakharov to emigrate with her husband and their two children, the Sakharov family said today.

Sakharov, winner of the Nobel Peace Price in 1975 for his support of human rights efforts in the Soviet Union, had complained recently that officials were harassing his stepdaughter and her husband "really as an attempt to get at me."

Today, the family reported that Yefrem Yankelevich, his wife, Tatyana, both 27, and their two children have been told to be ready to leave within 20 days.

Yankelevich said he had applied June 27 to emigrate to Israel. Under the Helsinki accords on human rights. Yankelevich's brother, David recently emigrated to Israel.

Western diplomatic sources suggested that the Soviets decided to permit the family to leave as one said, "because it enables the Russians to look nice just before Belgrade. They can say that 'even member of the family of well-known dissident Sakharov are permitted to leave'" The Soviet Union, the United States 33 other nations that signed the Helsinki agreement on European security are to meet this fall in Belgrade to discuss compliance with the human rights provisions of the accords.

Yankelevich told Western reporters today that he intends to settle in the United States. Four years ago, he was invited to study his specialty, radio the family reunification provisions of electronics, at Massachusetts Institute of Technology. He was refused a temporary exit visa.

The news that the Soviets were going to grant him and his family visas underscores the somewhat unpredictable nature of Soviet actions toward citizens it considers troublemakers.

In recent months, ever since the kremlin began arresting dissidents in a crackdown on human rights activism here, there have been several instances when the drive to throttle the voices seemed to pause and briefly abate. The combination of arrests and grants of permission of emigrate is steadily reducing the size and impact of the tiny disident community and its supporters.

Yanekevich apparently was singled out for harsh treatment after he accompanied Sakharov to a dissident's political trial in 1975. He was swiftly fired from his radio engineering job and his wife was expelled from her position on the journalism school of Moscow University.

Recently, authorities have questioned Yanelevich about a traffic accident in which he says he was not involved. Although he has not been an active voice among the dissidents, he also has been questioned about other dissidents. He said he refused to answer the queries.

The Yankelevich family is related to Sakharov by marriage. Tatyana Yanelevich is a daughter of Yelena Bonner, Sakharov's second wife, Sakharov's three children by his first marriage have publicly separated themselves from their father's human rights activities.