"This is the most hellish case I've ever seen," said Isle of Wight prosecutor Steve Edwards.
Molinski admitted to abusing infant daughters Ella and Piper, leaving them with fractured skulls, broken ribs and burned feet. Piper died from her injuries in February 2012.
In court just before sentencing, Molinski said "I'm sorry for all the pain I've caused".
Authorities arrested and charged Molinski with child abuse and murder, and they charged Webb with child abuse.
She admitted in court Wednesday that she knew Molinski would smoke spice before watching the girls while she went to work.
"As she admitted, she would do anything to maintain her relationship with Robert Molinski, and that was the trade that she made," Edwards said.
Webb said in court that she noticed the girls injuries, but did not take them to the emergency room.
Before the judge sentenced her to three years in prison, she said "looking back, I should have grabbed my children and run screaming down the street for help".
Once Webb is released, she cannot have any contact with Ella until she is 18 years old. Authorities did not know the girl's condition, but did say she is with Webb's family in Kentucky.