The singer, who was on the witness list, became the unexpected first witness to be called on by the prosecution in the case against William Balfour, who is accused of killing her mother, brother and 7-year-old nephew in October 2008.
"I told her over and over again to not marry him," Jennifer said, reports E! Online. "We did not like how he treated her."
Balfour is the estranged husband of her elder sister Julia Hudson, who shared a home with their mother in Chicago.
Jennifer broke down in tears when the prosecutor asked her about the last time she saw her family, which she revealed was at a surprise visit a week before they were murdered.
She said: "It was my mother, my brother, my sister and my dog. I was so surprised. The whole family is here. That was the last time I saw them. Thank God I had the opportunity."
Balfour is charged with first-degree murder in the deaths of Darnell Donerson, 57, Jason Hudson, 29, and Julian King, 7 along with home invasion, aggravated kidnapping, residential burglary and possession of a stolen motor vehicle.
Balfour pleaded not guilty to all charges.
During a court hearing in March, it was said that Balfour made at least two dozen death threats to the family before the murder.