Shonta Cathey also testified about the weapon prosecutors say was used in the shootings.
Jennifer Hudson and her sister Julia were back in the courtroom Friday for the dramatic testimony.
Shonta Cathey testified her then-boyfriend, William Balfour, admitted to her he shot Jennifer Hudson's mother and brother and then asked her to tell police he was with her.
Cathey, who testified she saw Balfour with the murder weapon before the shootings, said the night before and morning of the murders that she couldn't reach Balfour and tried calling him several times.
Cathey testified that Balfour told her he shot Jason Hudson after he charged at him, before turning his gun on Darnell Donerson.
She said the singer's nephew, 7-year-old Julian King, according to Balfour, was outside at the time of those shootings.
The day began with testimony from FBI agent Nikki Skovram, an expert in cell phone location analysis.
Skovram testified that on the morning Donerson and Hudson were killed, Balfour's cell phone was near their home around the time of the murders.
Using a PowerPoint presentation, agent Skovram showed how call records and the location of cell towers were used to determine the general vicinity of the phone, a radius of about four or five blocks.
Skovram said Balfour's phone was used for three calls that morning, including an 18-minute conversation that prosecutors say was with ex-wife Julia Hudson moments before the shootings.
Skovram said the phone then lost contact with the cell network for nearly four hours, saying it could have been the result of the phone being turned off.
Later, Jennifer Hudson laid her head on her sister's shoulder as 71-year-old Billy Graham testified he saw Balfour driving a white SUV on his block, not unlike the white suburban the singer's slain brother owned, and where King's body would be found.
Graham lives on the same block as Balfour's girlfriend Cathey and it was her apartment where Balfour was ultimately arrested.