The victim is the third daughter of late Konagala Apparao and his wife Devi of Tadepalligudem village, they said.
As Konagala died two years back, Devi, who is a labourer, through a neighbourhood woman kept her daughter in care of R Lakshmi and Gopi some three months back, while her other two daughters were admitted to an orphanage in Rajahmudry.
According to police, the couple used to frequently harass the hapless child. In a recent incident, which reportedly came to light on Tuesday, she was allegedly severely beaten up by the duo and sustained injuries.
Meanwhile, after being informed that her daughter was unwell, Devi rushed to the house of the couple and was shocked to see her condition, following which she rushed the child to a private nursing home and later to a hospital in district headquarters of Eluru, sources said, adding the victim is recovering.
Police said the foster parents and the intermediary, identified as Lakshmi, will soon be arrested.
Meanwhile, the district officials said the woman and her three children will be admitted to a local state-run charity home.