Oscar-winner Jennifer Hudson broke down in tears on Monday while testifying at the trial of the man accused of killing her mother, brother and 7-year-old nephew in a jealous rage four years ago.

Hudson was the first witness called after prosecutors and attorneys for William Balfour finished their opening statements. During her brief but emotional testimony, she told jurors her family didn't want her sister to marry Balfour and she spoke about the last time she saw her family members.

The singer and actress, who wore a subdued black dress and whose hair was wrapped in a bun, at first seemed composed as a prosecutor began asking her questions. But the testimony became increasingly difficult, and she began crying when talking about seeing her family the Sunday before the killings and later when a prosecutor showed her a picture of her mother.

Her voice also broke as she described her reaction when her sister, Julia Hudson, told her she was going to marry Balfour.

"None of us wanted her to marry him," she said, her voice cracking and struggling to hold back tears. "We did not like how he treated her," she said.

Balfour has pleaded not guilty to three counts of first-degree murder in the October 2008 slayings.

Early on during Hudson's testimony, a prosecutor asked her to identify the defendant. Hudson poked her head around the judge's bench and pointed at Balfour.

"He's sitting right there," she said.

Balfour looked back at Hudson. But he otherwise showed little emotion during her testimony and during opening statements.

After more than 30 minutes on the stand, Hudson grabbed a fistful of tissues and walked slowly across the courtroom directly in front of jurors. She then took a seat next to her fiance, David Otunga, best known for his stint on VH1's reality show "I Love New York."

A former federal prosecutor said prosecutors were shrewd to call Jennifer Hudson as their first witness.

"It rivets the jury," said Phil Turner, a Chicago attorney. "For better or worse it increases the importance of the case in jurors' minds."

Turner noted Hudson now can sit through the rest of the trial, in full view of the jury. Witnesses typically are not allowed to watch trials until they have testified.

Hudson, watching from a fourth-row bench Monday afternoon, bowed her head and dabbed away tears as prosecutors played a recording of the 911 call her sister made after discovering their mother's bloodied body.

"Oh my God, oh my God," Julia Hudson is heard yelling at a dispatcher, who tells her to stop screaming because he can't understand her. "My momma, my momma!"

Earlier, Julia Hudson testified that her ex-husband was so prone to jealousy, he even became angry when her son, Julian King, kissed her.

"He'd say, 'Get off my wife'," she said.

She described for jurors the first of many alleged threats by Balfour. After she rejected his pleas in May 2008 to reconcile, she said, Balfour grew agitated.

"He said, 'If you leave me, you will be the last to die. l'll kill your family first," she said, her voice quivering. She said he used precisely the same words at least several other times in the months before the triple homicide.

The couple's divorce was finalised last year.

The killings happened the day after Julia Hudson's birthday. Prosecutors say Balfour became enraged by balloons he saw at the home that he thought were from her new boyfriend.

Defense Attorney Amy Thompson offered jurors another take on the killings during opening statements, telling them that police pinned the slayings on Balfour because they felt pressured to make an arrest.

"As soon as that (that a celebrity was linked to the case) became known, they knew coverage would explode," Thompson said. "The police were on the hook. They had to find their man and find him fast."

Prosecutors say Balfour went inside the three-story house around 9 a.m. and used a .45-caliber handgun to kill Hudson's mother, 57-year-old Darnell Donerson, in the living room, and then shot her 29-year-old brother, Jason Hudson, twice in the head as he lay in bed.

He allegedly drove off in Jason Hudson's SUV with Julian inside. Authorities say he shot the boy in the head as he lay behind a front seat. His body was found in the abandoned vehicle miles away after a three-day search.

There are no known witnesses to the slayings, and it's unclear what physical evidence exists, including fingerprints or DNA. Prosecutors say gunshot residue was found on the steering wheel of Balfour's car. But the defense says it and other evidence is circumstantial.

During her opening statement, Thompson said DNA found on the gun and fingerprints found in the SUV didn't match Balfour's.

If convicted of at least two of the murder counts, the 30-year-old Balfour, on parole at the time of the killings after serving nearly seven years for attempted murder and vehicular hijacking, would face a mandatory life sentence. The trial could last up to a month.

Jennifer Hudson, the 2004 "American Idol" finalist and 2007 Oscar winner for her role in Dreamgirls, is expected to attend each day of testimony. Judge Charles Burns has instructed jurors to set aside any sympathy for Hudson and decide a verdict strictly according to testimony.