Hudson arrived in court just after 11 a.m. Her hair was pulled back, and she wore a black top and olive skirt. She sat between her sister Julia and her fiance in the fourth row.
Balfour was estranged from Julia Hudson at the time of the killings. He has pleaded not guilty to murdering Hudson's mother, brother and 7-year-old nephew.
Because no one witnessed the murders, prosecutors must build a circumstantial case. They got down to business, introducing photos and video as evidence.
The first witness on the stand Tuesday was Sgt. Richard Dowling of the Chicago Police Department. Jennifer Hudson hung her head and shut her eyes while the officer described finding her brother dead in his bed of a gunshot wound to the head. His sheets were pulled up as if he'd been sleeping. Dowling also described finding her mother's body sprawled in the living room with gunshot wounds through her back.
Jurors also heard from acquaintances of Balfour who placed him at the murder scene and a half brother of the Hudson family.
Abudullah Kareem Smith told the jury that while Balfour told him he wanted to hurt Jason Hudson, he never heard him make any threats to Julia Hudson.
Tyrone Dunbar, another neighbor, testified that he bought drugs from Balfour hours after prosecutors say Balfour committed the murders.
Tape from a nearby store shows Balfour getting out of the car with Dunbar, and Dunbar asked if he was carrying a gun. "He said he was dirty but didn't say anything about no gun," Dunbar said.
Prosecutors said the murder weapon was a gun that Balfour stole from Jason Hudson. Jennifer, Julia and Jason Hudson's half brother, Lonnie Simpson, testified that he bought Jason that gun for his protection.
A former forensic investigator with the Chicago Police Department pointed to photos of the dead victims. Before they were shown, Jennifer Hudson left the courtroom so she would not have to see them.
Emotional testimony from Jennifer Hudson
Jennifer Hudson was the first to take the stand Monday. At times, she was holding a tissue and told the jury that none of her family members wanted her sister Julia to marry Balfour. He is charged with murdering Jennifer Hudson's mother, brother and nephew out of jealousy. Jennifer Hudson's brother Jason was a drug dealer who had been shot before.
Jennifer Hudson testified that she tried to keep her distance from Balfour and didn't like how he treated her sister. Julia Hudson also testified about Balfour's alleged threats and her 911 call after finding her mother's body.
Balfour's defense pointed out that Julia Hudson never reported the alleged threats. Defense attorneys noted that DNA on the handgun used in the murders didn't match Balfour.
On Tuesday, prosecutors are expected to introduce cell phone records and security camera footage that place Balfour in the area at the time of the killings.
Jennifer Hudson entered through a back door like she did Monday in order to avoid the cameras in front of the courthouse and in the lobby at 26th and California.
The Associated Press contributed to this report.