Robert Molinski's girlfriend, Mary Leanne Webb, received a three-year sentence Wednesday for failing to protect her daughters, The (Norfolk) Virginian-Pilot reported.
Webb, 24, of Smithfield told the court she became suspicious when she saw bruises on the two infants, Piper and Ella, who were 4 months old when Piper died in February 2012.
Ella survived and is living with relatives of Webb.
Webb said she decided to believe Molinski when he denied harming them because she did not want to end the relationship.
"Looking back, I realize I should have grabbed my children and gone running down the street for help," she said, weeping. "I'm sorry."
Prosecutors said Molinski, 23 -- who cared for the children while Webb was working -- used synthetic marijuana.
His lawyer said Molinski had been abused as a child.
"Mr. Molinski simply had no parenting skills," Deputy Public Defender Antoinette Tucker said. "He certainly should have never been put in a position to care for these babies."