"I don't care who you are, just let the baby go," Julia Hudson said in a press conference held at the Pleasant Gift Missionary Baptist Church on Saturday. "I just want my son. He don't deserve this."
An Amber Alert was issued for the boy, Julian King, 7, on Friday and named Julia Hudson's estranged husband, William Balfour, 27, as a suspect in the slayings. Balfour is in police custody for questioning, but no charges have been filed.
An autopsy confirmed that Darnell Donerson, 57, and Jason Hudson, 29, died of gunshot wounds.
"It hurts," Julia Hudson said of her mother and brother's deaths. "We're in a state of shock. I don't know what else to do but pray."
With the boy's father, Greg King, by her side, Julia Hudson asked the public to keep an eye out for him, mentioning that he responds to nicknames like "Juice Box" and "Dr. King." "He's out there. Just let him go," she said. Addressing her son, she added: "I love you. Mama's looking for you."
Julia added that sister Jennifer, 27, the Oscar-winning star of Dreamgirls and an American Idol alum, had flown in to be by her side as soon as she heard the tragic news. She said that upon hearing the news, Jennifer, "was screaming" and "flew in right away."
Julian had been at his grandmother's house on Friday and the two were, Julia said, "extremely close."
"I worked, so he was always with her, day in and day out," she said. "She did everything for him. She got him ready for school. She got him ready for bed."
The Chicago FBI office confirmed to PEOPLE on Sunday that the agency is involved in the search for the boy.
When asked whether they had any promising leads, Ross declined to comment. "The key word there is assisting. It is still a Chicago police investigation, both the murder and the kidnapping, and any announcements would be best to come from them."
And on Sunday morning, Jennifer Hudson updated her MySpace page with recent photos of her nephew and a statement from her family: "Thank you all for your prayers and your calls. Please keep praying for our family and that we get Julian King back home safely. If anyone has any information about his whereabouts please contact the authorities immediately. Here is a picture of Julian and what he was last seen wearing. Once again thank you all for being there for us through this tough time."
Reporting by NINA BURLEIGH