Bruce Zidarich said he tried to avoid being alone with Stacy so as not to antagonize Drew.
"She couldn't have any friends, no other friends besides family or Sharon the neighbor," Zidarich said. "She couldn't have friends that were guys, for sure. He would keep tabs on her every move."
Cassandra Cales told ABC News she told her sister to stand up to her husband.
"It wasn't right," Cales said. "I told her, 'You need your friends, you know, different personalities and smiles. You have to be able to get out and have fun. Just because you're out doesn't mean you're cheating, you know, and he's gotta learn.'"
When Kathleen Savio died in March of 2004, friends said they grew increasingly alarmed.
"We never really talked about that with [Stacy]," Zidarich told ABC News. "Between the other relatives, you know, we all figured he did it."
Drew Peterson has denied any involvement with Savio's death or Stacy's disappearance. In an interview on NBC's "Today" earlier this week, Peterson said both women were mentally unstable.
Savio "came from an abusive life," Peterson said. "After she had children, hormones kicked in -- and again, [it was] an emotional roller coaster with her."
Numerous calls to Drew Peterson and his relatives seeking comment for this article were not returned.
As Stacy's marriage with Peterson progressed, according to friends and family, Stacy became increasingly isolated and unhappy. She would have left Drew Peterson long ago, said Cassandra Cales, if it wasn't for her children -- Anthony, 4, and Lacy, 2.
She also cared for two children Peterson had with Kathleen Savio.
"They were the whole reason she stayed in that relationship," Cales said of the children, all four of whom are currently staying with Drew Peterson, according to news reports. "All she cared about was those kids."
On Oct. 17, 11 days before she disappeared, Stacy e-mailed a friend, Steve Cesare, with concerns about her marriage, according to the Chicago Sun Times.
"i have been arguing quite a bit w/my husband," Stacy wrote in the message, which Cesare gave to the Naperville Sun. "as i mature with age i am finding that the relationship i am in is controlling, manipulative, and some what abusive."
She continued, "if you could keep me in your prayers i could use some wisdom, protection, and strength."
It was just before she went missing, friends say, that Stacy decided she was going to tell Peterson of her intention to leave.
"She called me on Thursday," Pamela Bosco told ABC News, "and my advice to her was get a lawyer, do it properly, you have children involved. Make sure you don't jeopardize that.
Cales said she spoke to her sister on Friday night, just two days before she disappeared.
"She told me Friday night, 'If anything happens to me, I fear for my life,'" Cales said.
Stacy Peterson hasn't been heard from since she went missing on Sunday, Oct. 28.
Since that day, police have searched forests, fields and waters in the Bolingbrook area for clues in the case. On Saturday, friends, family members and other sympathizers staged a vigil for her, walking from Kathleen Savio's former home to the house Stacy and Drew Peterson shared, some carrying candles and signs.
On NBC's "Today" earlier this week, Drew Peterson, who was suspended without pay from the Bolingbrook Police Department this month, said his wife had asked him to end the marriage many times before she vanished.
"Stacy would ask me for a divorce after her sister [Tina] died on a regular basis," he said. "I'm not trying to be funny here, [it was] based on her menstrual cycle."
Zidarich expressed regret at the way Stacy Peterson decided to end her relationship with Drew.
"I just wish she would have planned it a little better," he told ABC News, "that she would've gotten out of the house before she told him."