UPDATED 04/23/12 8:01 p.m.

CHICAGO (CBS) — Oscar winning actress and singer Jennifer Hudson took the stand Monday in the trial of the man accused of killing three members of her family, providing an emotional start in the case against her sister’s ex-husband, William Balfour.

Hudson, wearing a black dress, maintained her composure through most of the testimony against the man who prosecutors say killed her mother, brother and nephew, although she did break down into tears at one point.

Her sister, Julia Hudson also testified Monday, telling jurors about Balfour’s alleged threats against the family.

Prosecutors say Balfour killed Darnell Donerson, Jason Hudson and Julian King in a fit of jealous rage on Oct. 24, 2008, believing Julia was cheating on him.

Jennifer Hudson did not testify about the shootings themselves, but did provide emotional testimony that she and her family did not want Julia to marry Balfour, because they didn’t like how he treated her.

READ: Full transcript of Jennifer Hudson’s testimony

“None of us wanted her to marry him,” said an emotional Hudson. The two married anyway, although neither Jennifer Hudson, nor her mother or brother attended.

Jennifer Hudson said she did not find out about the marriage until a couple months later. She also said she tried to avoid Balfour whenever she could, despite his relationship with Julia.

“I tried to keep my distance with William any chance I got. Where he was, I tried not to be. So if I saw him, I separated myself,” she said.

Jennifer Hudson later watched from the gallery when her sister, Julia, took the stand, and told jurors about Balfours alleged threats against her and her family, as well as his fits of jealousy.

Julia Hudson said Balfour was so jealous of anyone else’s attention to her that he got angry when Jennifer Hudson sent her sister gifts, and when Julia’s 7-year-old son, Julian King, kissed her.

“Julian couldn’t kiss me. (He’d say) ‘Don’t kiss my wife.’ Julian couldn’t lay up under me. ‘That is my wife. Get up off my wife.’ He was very jealous of Julian,” she testified.

LISTEN: WBBM Newsradio’s Bernie Tafoya reports

She also said that Balfour got upset about her sister’s gifts, because he didn’t buy it. He didn’t provide it. I guess I was showing gratitude to somebody else, and not him, so it was a problem.”

Julia Hudson and Balfour separated in February 2008, but continued to have a sexual relationship for months afterward. She testified that he also repeatedly threatened her, pointing to one time at a McDonalds when he asked to get back together and she said no.

She said Balfour told her, “If you leave me, you will be the last to die, I’m going to kill your family first.”

But defense attorneys tried to poke holes in Hudson’s claims about the threats, noting she never told police about them. She also admitted she continued a sexual relationship with Balfour even after his alleged threats.

She said she never reported any of the alleged threats to police because “I didn’t believe him.”

Julia testified she found her mother’s body in the living room of their home on Oct. 24, 2008, after she got home from work. At first, she thought it was her son lying on the floor, but when she got a closer look, she saw it was her mother lying in a pool of blood.

“I left. I was running, I was screaming, I was hollering somebody please help mee. Somebody killed my mother. Call the police. Somebody help me,” she testified.

Police later found her brother Jason’s body in the bedroom, shot twice in the back of the head. Her son, Julian King, was not found for three more days, when a man walking his dog spotted Jason Hudson’s stolen SUV and reported it to police. Julian’s body was inside, covered in a shower curtain. He’d been shot twice in the head.

Prosecutors claim that Balfour killed the Hudson family members in a jealous rage after he showed up at her home the morning after Julia’s birthday. He saw some birthday balloons that Jennifer had sent to Julia, and believed they were from another man.

Assistant Cook County State’s Atty. Veryl Gambino said Balfour repeatedly threatened to kill Julia Hudson’s family, and then her.

“They weren’t just words. They weren’t just threats. They were deadly warnings of what was to come,” Gambino said.

But defense attorneys claimed police did not properly investigate the killings, including evidence that Jason Hudson was a drug dealer who’d been shot before, and that his drug ties could have played a role in the deaths.

Assistant Cook County Public Defender Amy Thompson also pointed out that DNA and fingerprint evidence in the case did not link Balfour to the murder weapon, or the SUV where Julian King’s body was found.

“Each person’s DNA absolutely identifies them and it’s been a tool that the police department has used to great effect over decades,” Thompson said. “In this case, they found DNA on that murder weapon….it showed that a male handled that weapon … but it showed absolutely, positively that it did not identify William Balfour.”

“We don’t know who that DNA belongs to. They never found out. But we know it’s not William,” she added.

Thompson claimed, because of Jennifer Hudson’s fame, authorities rushed to arrest someone for the deaths and focused exclusively on Balfour.

“They had to find their man and find him quickly. They got one name out on that street and from the moment they got that name they have been spending their time building a case rather than investigating a crime. That man’s name was William Balfour,” she said.

Thompson said police and prosecutors failed to follow evidence that Jason Hudson was a drug dealer, and that his drug connections could have played a role in the deaths.

“Jason Hudson was a drug dealer,” she said, adding that, before the murders, Jason Hudson was shot two other times over drugs.

“He was involved in a dangerous business and it brought danger to him and he lived in Englewood, probably the most dangerous neighborhood in the city of Chicago,” Thompson said. “(Police) did nothing to determine if it might have been Jason Hudson’s violent business that led to him getting shot twice before, that led to these murders. … They did nothing to follow the trail other than the weak trail that leads to William Balfour.”

Jennifer Hudson is expected to be in court throughout Balfour’s trial, which is expected to last about a month.

Outside the courthouse, metal barricades lined California Avenue right outside of the courthouse. Media from all over the country – and even fans – are expected to show up for the month-long murder trial.

“The bottom line is, everyone’s concerned about courtroom decorum,” CBS 2 legal analyst Irv Miller says. “They want this to be a trial and not a media circus.”

Hudson is not going through the front entrance to the courthouse, but is instead being brought in on the side of the building through an entrance normally used by judges.

“She’s going in the back door, and she’s going to be brought up through a separate elevator up to the fifth floor,” Miller said.

According to Miller, Hudson was called to testify “as a life-and-death witness, that is, the family members were alive, and then they were deceased. That’s a crucial element to murder cases.”

Because that was the only goal of Jennifer Hudson’s testimony, she will be allowed to sit in on the remainder of the trial.

Testimony is scheduled to resume Tuesday morning.

–Todd Feurer, CBS 2 Web Producer