William Balfour, who was married to Jennifer's sister Julia, is charged in the 2008 shooting deaths of the sisters' mother and brother and Julia's 7-year-old son.
ABC News said Jennifer Hudson, an Oscar-winning actress and Grammy Award-winning singer, tearfully testified about Julia's stormy relationship with Balfour when she took the stand at the opening of his trial.
"None of us wanted her to marry William," Jennifer said. "We did not like how he treated her."
WLS-TV, Chicago, said Jennifer spoke for about 50 minutes, recalling how close she was to her family and how devastated she felt as she identified the bodies of her loved ones at the morgue.
Julia painted a portrait of Balfour as a jealous and unfaithful husband who "sold drugs" for a living and repeatedly threatened to kill her and her family, WLS-TV said.
"If you leave me, you will be the last to die," Julia said Balfour told her. "I'm going to kill your family first."
However, Balfour's lawyers said Julia never reported the alleged threats to police.
Balfour's legal team also said Hudson's late brother Jason, one of the victims, was a drug dealer, who had been shot at least two times in the past, suggesting other people might have had reason to harm him.
Balfour is charged with three counts of first-degree murder. His trial is expected to last a month.