Magdalena Lopez, 30, was arrested Tuesday night after police, responding to her 911 call, found 9-year-old Antonio Lopez and 2-year-old Erik Lopez dead in separate rooms of the family's home in Dyer, 10 miles southwest of Gary.
A police report said that as officers approached the house, the woman walked out with blood stains on her clothes and bare feet.
According to the report, she told officers, "I had to kill them. They're in a much better place now."
An autopsy determined the boys died of massive skull fractures caused by the metal weight.
Irene Lopez said her daughter-in-law was diagnosed with bipolar disorder within the past year.
"She'd been sick for a good six months," Irene Lopez said. "We took her to specialists. She was hospitalized once. We thought she was doing better, but she fooled us. Only God knows why this happened."
The woman was being held at the Lake County Jail without bond. A prosecutor said she would likely have her first court appearance Friday.
The boys' father, Robert Lopez, was at work in East Chicago when he learned about the deaths, said Dyer Police Chief Richard Quinn.