Oscar-winner Jennifer Hudson broke down in tears Monday while testifying at the trial of the man accused of killing her mother, brother and seven-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.

'None of us wanted her to marry him. We did not like how he treated her'—Jennifer Hudson

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.

Jennifer Hudson testified the family did not want her sister to marry William Balfour. (Associated Press)

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.

Julia Hudson testifies

Julia Hudson testified in Monday afternoon, confirming that her family disapproved of her relationship with Balfour. Jennifer Hudson watched from a fourth row bench. Julia Hudson testified that Balfour was so prone to jealousy, he even became angry when her son gave her a kiss.

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

Prosecutors gave jurors their first glimpse of the gun allegedly used in the killings. Julia Hudson said it was her brother's gun that Balfour allegedly stole months before the slayings.

During opening statements, prosecutor Veryl Gambino said Balfour began threatening to harm the family after moving out of Hudson's family home. Balfour's divorce from Julia Hudson was finalized last year.

"Those were not idle threats. ... They were deadly warnings of what was to come," Gambino told the Cook County jury.

Defence attorney Amy Thompson offered jurors another take on the killings, telling jurors police pinned them 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."

Balfour made threats

Lead prosecutor James McKay has said that the day before the attack, which was Julia Hudson's birthday, Balfour had threatened her, saying, "If you ever leave me, I'm going to kill you, but I'm going to kill your family first." She didn't take him seriously, McKay said, because Balfour hadn't acted on the threats before.

Prosecutors say Balfour became enraged by balloons he saw at the home that he thought were from Julia Hudson's new boyfriend. She told investigators that on the day of the killings, she saw Balfour linger behind as she was driving off from home to her job as a school bus driver.

Prosecutors say Balfour went back 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 defencesays 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.