HALLANDALE, Fla. (AP) _ A relative of fashion designer Albert Nipon's brother and sister-in-law says the couple were strangely nervous and upset in the weeks before they were slain in their luxury condominum.

Edward Nipon, 58, and his wife, Sylvia, 67, who lived in Philadelphia and wintered here, were found to shot to death in the living room of their oceanfront home Saturday in a building secured by a guard and surveillance cameras.

Police on Sunday said they were searching for a young couple who had talked to the Nipons hours before they were found slain.

''Apparently they were just sitting there engaged in conversation,'' said Hallandale Police Chief Richard Fox. ''I strongly feel it was someone they knew. There's nothing that leads us to believe it was random.''

Nipon was the youngest of four brothers who parlayed a maternity clothes company into an iternational women's-wear design and manufacturing firm bearing the name of Nipon's older brother, Albert.

Although retired from the business, Edward Nipon remained active while his brother was imprisoned at Eglin Air Force Base on tax-evasion charges.

In May 1985, Albert Nipon was sentenced to a three-year prison term after he admitted bribing federal agents with $215,000 and cheating the government out of more than $1.5 million in taxes. He was released to a halfway house in Philadelphia last week.

Leon Nipon, Albert Nipon's 30-year-old son, said his uncle had been ''upset and pensive'' and his aunt ''very uptight'' over the last several weeks. He said the family did not know why.

Fox said a young couple had been seen talking to the Nipons shortly after midnight Friday for about 15 minutes.

''Mrs. Nipon had gone down into the lobby several times, apparently expecting somebody, and sometime approximately 10 minutes after 12, there was a young couple that came in,'' Fox said. ''They went up to the apartment. He (Nipon) escorted them out.''

The door to their apartment was unlocked but nothing appeared to have been taken from their home, the chief said. No one heard the shots, police said.

Neighbors said the Nipons were cautious about admitting visitors and locked their door at night.

''Mr. Nipon had three locks on the door,'' said Louis Auslander. ''It's not like them to leave the door open. I really believe that somebody did a number on them, and it must have been somebody they knew.''