Jennifer Hudson breaks down while testifying at murder trial

Jennifer Hudson performs in memory of the late Whitney Houston at the Staples Center during the 54th Grammy Awards in Los Angeles, California, February 12, 2012. AFP PHOTO Robyn BECK (Photo credit should read ROBYN BECK/AFP/Getty Images)
Jennifer Hudson performs in memory of the late Whitney Houston during 54th Grammy Awards in Los Angeles, Feb. 12, 2012.
Robyn Beck

(CBS/AP) CHICAGO - Jennifer Hudson struggled to hold back tears 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.

Pictures: Jennifer Hudson testifies in murder trial
Complete coverage of the Hudson family murders on Crimesider
Pictures: Jennifer Hudson

Hudson was the first witness called after prosecutors and attorneys for William Balfour finished their opening statements. Hudson testified for about 30 minutes Monday at the start of the murder trial. She started answering questions calmly but became emotional the longer it went on.

The singer and actress, who wore a subdued black dress and whose hair was wrapped in a bun, responded to prosecutors' questions softly but confidently and when asked to identify the defendant, poked her head around the judge's bench and pointed at Balfour.

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

Her voice broke, though, as she started to describe 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. Prosecutors allege that Balfour killed the victims in a jealous rage because his estranged wife had a new boyfriend.

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.

Hudson, the 2004 "American Idol" finalist and 2007 Oscar winner for her role in "Dreamgirls," is expected to attend each day of testimony. It could last up to four weeks.

She was joined on Monday by her fiance, David Otunga, and other family members.

Professionally, Hudson has had a busy month so far. She recently performed on "American Idol" and also sang at The Revlon Concert for the Rainforest Fund at Carnegie Hall, in New York. She appears on the soundtrack to the new film, "Think Like a Man."