The unidentified woman, 43, whose trial was to begin Monday, agreed to plead guilty to reckless endangerment and causing harm to a child, and faces up to 25 years in prison.
Her stepdaughter, 15, was found wandering neighborhood streets, barefoot, thinly dressed and weighing 68 pounds in February 2012, after escaping from the locked basement where she was kept, the Milwaukee Journal Sentinel reported.
A doctor examined her and diagnosed her condition as "serial child abuse with prolonged exposure to definite starvation."
Last month the girl's father was found guilty of child neglect, and her stepbrother, charged with sexual assault and child abuse, will be tried in June, the newspaper said.
Interpol investigating stolen passports on missing flight
Senate Democrats to pull all-nighter on climate change