A D.C. Superior Court judge blocked the sale yesterday of the house of two slain South Vietnamese diplomats after a request from their son, who is their accused killer, and his sister Madame Ngo Dinh Nhu, the so-called Dragon Lady during the Vietnam War.

Nhu, the sister-in-law of assassinated South Vietnamese President Ngo Dinh Diem, said in a letter filed in court that her parents' Northwest house should be placed in the hands of a guardian until their deaths are resolved and to "appease their souls."

The action by Judge Iraline Barnes is part of a drawn-out dispute over the wills of Tran Van Chuong, 88, a former South Vietnamese ambassador to the United States, and his wife Nam-Tran Chuong, 75, Vietnam's permanent observer to the United Nations until she and her husband resigned their posts in the early 1960s to protest Diem's brutalization of Buddhists.

Their son, Tran Van Khiem, 60, is awaiting trial in their deaths a year ago and has challenged the validity of a will that disinherits him. The judge's action came after a second sister and court-appointed administrator of the estate, Lechi Tran Oggeri, said she should be able to sell the house to a buyer willing to pay about $370,000.

Khiem's and Nhu's attorney, Thomas Mauro, said that the house, at 5601 Western Ave. NW, should not be sold without Khiem's consent as the head of the family for religious and cultural reasons.

Nhu wrote in her letter, mailed from Rome, that the house is "where our parents lived their last years and had a death which must be cleared in order {for} justice and honor to be rendered to their names and thus appease their souls."

In her order, the judge said she was blocking the sale because Oggeri had not received the court's approval, as required, to sell the house.