The brother of one of the American nuns murdered two years ago went to a dusty courthouse in the rural town of Zacatecoluca today to request that a judge there press his investigation beyond the five National Guard enlisted men now under indictment.

William P. Ford, a New York lawyer whose sister Ita was killed along with two other nuns and a lay worker on Dec. 2, 1982 was accompanied by members of the Lawyers' Committee for International Human Rights. The delegation is one of several groups arriving here prior to the expected certification by the Reagan administration, later this month, that human rights conditions are improving.

Ford cited in his formal declaration as "an injured party" what he called "certain statements in the court record and other facts which indicate that the five defendants were acting pursuant to superior orders or at the direction of others who may share responsibility for the crime." Under Salvadoran law, a family member can ask information of the court.

Since the day the churchwomen's bodies were exhumed from a shallow grave near Zacatecoluca there has been widespread suspicion, sometimes openly expressed by U.S. officials, that the killing was ordered high up in the government or covered up by more senior members of the armed forces. But in the months since the five guardsmen were arraigned, U.S. and Salvadoran officials have insisted that no valid leads have been found to support such theories.

Ford cited several issues that he said could be pursued. He noted an Aug. 9, 1982 statement by one guardsman indirectly involved that the subsergeant in charge of the accused squad had said he was acting under orders to detain the women. Ford said there has been no apparent effort to interrogate each of the subsergeant's superiors in detail on this question.

No investigation has been made as to whether an order allegedly required by the squad to operate outside their usual jurisdiction was issued, according to Ford. He also said that the subsergeant's immediate superior has testified that the subsergeant confessed to him and that he advised the subsergeant to inform the director of the guard.

Judge Bernardo Rauda Murcia reportedly said that if legally presented evidence on these and other points is submitted he would accept it.

Yesterday, an appeals court ordered the case back to the First Penal Court of Zacatecoluca, 25 miles southeast of San Salvador, a move that delayed the trial for at least a month.

The others slain were Maura Clarke, another Maryknoll nun, of Belle Harbor, N.Y.; Ursuline nun Dorothy Kazel of Cleveland, and Catholic social worker Jean Donovan of Stamford, Conn.