CHICAGO â€"  A gift of balloons. That, prosecutors contend, is what sent singer Jennifer Hudson's then brother-in-law into such a jealous rage that he shot dead her mother, brother and 7-year-old nephew in a horrific act of vindictiveness in the home where the Hollywood star grew up.

A prosecutor is expected to tell jurors that Monday during opening statements at the trial of William Balfour -- charged with three counts of first-degree murder in the Oct. 24, 2008, slayings that generated nationwide headlines because of Hudson's fame.

The estranged husband of Hudson's sister, Balfour believed the balloons he saw at the Hudson home came from Julia Hudson's new boyfriend; driving away for her job as a school bus driver, she glimpsed Balfour in her mirror still lingering outside, prosecutors have said.

They 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 -- 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 Julia Hudson's son, Julian King, inside. Authorities say he shot the boy nicknamed Juice Box 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, be it fingerprints or DNA. Prosecutors say gunshot residue was found on the steering wheel of Balfour's car. But the defense says and other evidence is circumstantial.

A gun, which Balfour allegedly stole months before from Hudson's brother, was recovered in a lot near where the SUV was found and will be presented as the murder weapon.

Establishing motive may pose less of a challenge.

A high-school dropout and one-time Gangster Disciple known by the gang name "Flex," Balfour allegedly threatened to kill the Hudson family at least two dozen times, starting earlier in 2008 when he moved out of the house, lead prosecutor James McKay has said.

A day before the killings, on Julia Hudson's birthday, McKay says, Balfour told her, "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.

Court records indicate the now 34-year-old Julia Hudson's divorce from Balfour was finalized last year.

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.

Jennifer Hudson's name is among 300 on a list of potential witnesses, though it's not certain she will testify. The 2004 "American Idol" finalist and 2007 Oscar winner for her role in "Dreamgirls" is expected to attend each day of testimony, which could last up to a month.

Judge Charles Burns has instructed jurors to set aside any sympathy for Hudson and decide a verdict strictly according to testimony. The panelists include a teacher, a trucker and two people who have had close relatives murdered.

Prosecutors have said Balfour claimed he wasn't near the Hudson home at the time of the killing, but they are expected to introduce cellphone records that allegedly prove he was in the area when two teenage girls who live nearby heard gunshots.

The witnesses didn't immediately report the shots to police because the sound of gunfire isn't uncommon in the impoverished, crime-ridden Englewood neighborhood, according to court filings.

The trial is being held in a nearly century-old, oak and granite courtroom where several TV legal dramas have been filmed. It's the same courtroom where R&B singer R. Kelly was acquitted on child pornography charges in 2008