Jennifer Hudson Family Murder Trial: Actress tesifies in the murder trial of William Balfour

(CBS/AP) CHICAGO - Jennifer Hudson says her family didn't want her sister to marry the man now accused of killing her mother, brother and 7-year-old nephew in October 2008.

CBS Chicago reports prosecutors called singer Jennifer Hudson as their first witness on Monday in the murder trial of William Balfour.

Hudson told jurors that when she moved out of her mom's house in 2006, her sister Julia was dating Balfour. Hudson's voice broke as she testified on the stand about how she reacted when her sister told her she was going to marry the defendant.

Prosecutors say he killed the Oscar winner's family after becoming enraged that Hudson's sister, his estranged wife, had begun dating someone else.

She testified she last saw her mother, brother and nephew the weekend before the murders at a family gathering.

Hudson reportedly held back tears as she told jurors, "None of us wanted her to marry him." She added, "We did not like how he treated her."

Hudson is expected to be in court throughout the trial of her ex-brother-in-law.

Earlier, prosecutors began their opening statements, telling the jury that Balfour told Hudson's sister: "If you leave me I'll kill you. I'll kill your family first."

Balfour's defense team began its opening statements shortly after noon. They said the gun from the murders were tested for DNA but didn't match him. Further, they said, Julia never reported any domestic problems with Balfour.

Hudson's sister Julia is a key witness and there is a slight chance she'll be called up today. She is expected to tell the jury that Balfour had threatened to kill her family.

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

Hudson previously has declined to talk publicly about the killings.

Complete coverage of the Hudson family murders on Crimesider