William Balfour, 30, is charged with three counts of first-degree murder. He has pleaded not guilty.
Struggling to hold back tears, Ms Hudson said on the opening day of the trial that her family had not wanted her sister Julia to marry Mr Balfour.
Mr Balfour is accused of killing Ms Hudson’s mother, brother and a nephew.
Darnell Donerson, 57, and Jason Hudson, 29, were found shot dead in their Chicago home on 24 October. The body of seven-year-old Julian King was found days later.
Mr Balfour was arrested shortly after the killings.
“None of us wanted her (Julia) to marry him,” Ms Hudson told the Cook County court on Monday.
“We did not like how he treated her,” she added.
Ms Hudson was the first witness called to the stand after prosecutors and attorneys for William Balfour finished their opening statements.
Hundreds of potential witnesses could also be called to testify during the trial which is expected to last at least a month.
If convicted, Mr Balfour faces a life sentence.
The prosecution alleges that he shot the three in a jealous rage because he was upset that his estranged wife had been seeing another man.
Ms Hudson, a finalist on the 2004 series of American Idol, won an Oscar in 2007 for her supporting role in the musical Dreamgirls.
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