CHICAGO, Illinois (CNN) -- The sister of Oscar-winning actress Jennifer Hudson made an emotional appeal Saturday for the safe return of her 7-year-old son, who disappeared after a double shooting Friday on Chicago's South Side.
"All I ask, I don't care who you are, just let my baby go, please," Julia Balfour, Hudson's sister, said at a news conference Saturday night. "I just want my son."
Hudson's mother, Darnell Donerson, and brother, Jason Hudson, were shot to death Friday in their home. The child, Julian King, has been missing since.
Hudson was said to be in Chicago on Saturday night but did not attend the news conference. The missing child's father, Greg King, attended but did not speak publicly.
Earlier Saturday, Julia Balfour's mother-in-law said that police had questioned her son, William Balfour, in the slayings.
Michele Davis-Balfour insisted that her son had nothing to do with the shootings and urged the public to focus on finding the missing boy.
"Out of no means did my son do this. This heinous crime to this family is unbelievable. It's unbearable," Davis-Balfour told WLS in Chicago.
"Whoever has Jennifer Hudson's nephew or Julia Balfour's son, please, I'm begging you, me and my family are begging you to return that son." Watch Davis-Balfour's emotional appeal »
Police issued an Amber Alert for Julian and asked residents to look out for a 1994 white Chevrolet Suburban with Illinois license plate X584859.
Julian is described as being 4 feet, 11 inches tall and weighing 130 pounds. He was wearing a striped brown polo shirt and khaki pants when last seen, authorities said in the Amber Alert.
WLS reported late Friday that a suspect, whom it now identifies as William Balfour, was in custody but that the boy was not with him when he was detained.
Chicago police spokesman Daniel O'Brien would not comment on whether a William Balfour was in custody or whether police had questioned a man by that name.
"Police have and continue to speak with numerous" people, O'Brien said. He said no charges had been filed against anyone in connection with the murders. Watch the police news conference »
William Balfour says on his MySpace page that he is married to Jennifer Hudson's sister Julia and has a stepson.
"I might as well let you all know that Jennifer Hudson is my wife's sister I'm proud of her and wish her nothing but the best in what she do," he wrote.
According to the Illinois Department of Corrections, Balfour, 27, is on parole. He spent nearly seven years in prison for attempted murder, vehicular hijacking and possessing a stolen vehicle.
His MySpace page also says he lives on Chicago's South Side.
Deputy Police Chief Joseph Patterson said the bodies were found about 3 p.m. Friday (4 p.m. ET), when a relative arrived and found the body of a woman on the living room floor. The relative backed out of the house and called police, Patterson said. Authorities found a man shot to death in a bedroom.
The Cook County Medical Examiner's Office said Saturday that Donerson and Jason Hudson suffered multiple gunshot wounds and ruled the deaths homicides.
The FBI is working with Chicago police to find Julian, Chicago Police Superintendent Jody Weis said. There is no evidence that the boy has been taken across state lines, which would make the abduction a federal offense. The action is being taken as a precaution because it would be possible to cross state lines in the amount of time the boy has been missing, he said.
Neighbors reported hearing gunshots earlier Friday, Patterson said. Authorities found no signs of forced entry to the home and were not sure whether other items were missing. Watch the scene outside the house »
"You've got two people who were killed inside a home. That alone will produce a great deal of evidence," Weis said of forensic evidence at the crime scene.
Hudson won a Best Supporting Actress Oscar for her portrayal of Effie in the film version of the Broadway musical "Dreamgirls." She competed on the third season of "American Idol" in 2004, making it to the top seven contestants before being eliminated from the contest.