(CBS News) CHICAGO -- It was an emotional day in a Chicago courtroom for Oscar-winner Jennifer Hudson Monday.
She took the witness stand to testify against her former brother-in-law.
Thirty-year-old William Balfour is accused of fatally shooting Hudson's mother, Darnell Donerson, her brother, Jason Hudson, and her seven-year-old nephew, Julian King.
Prosecutors say it was an act of spite by Balfour after Hudson's sister, Julia, demanded a divorce from him.
On Monday, as the first witness for the prosecution, Jennifer Hudson testified for about half-an-hour, choking back tears as she identified pictures of her mother, and recalling for the jury how opposed the family was to Julia's marriage to Balfour.
"We did not like how he treated her," Hudson testified.
In January, on the CBS program "The Talk," Hudson spoke of trying to recover from the murders, though the memory of her loved ones is still vivid.
"I hear their voices in my head every day," she said.
The defense is arguing that the murders may have been drug-related and carried out by someone else.
But Hudson's sister Julia testified Monday that Balfour once told her, "If you leave me, you will be the last to die. I'm going to kill your family first."
Prosecutors also argued that Balfour was enraged on the day of the murders when he saw birthday balloons in the family home, suspecting they'd been sent to Julia by another man.
In fact, they'd been sent to Julia by her sister -- Jennifer Hudson.
To see the Dean Reynolds report, click on the video in the player above.