Authorities said they were called to Regina Bennett's home in the Olney neighborhood Friday by neighbors who said they were being threatened by the woman.
Police arrested Bennett, but neighbors warned police there was a young child living in the woman's home, WTFX-TV, Philadelphia, reported Sunday.
Upon searching the house, police found Bennett's cousin, Sakinah, whose last name was not reported, WPVI-TV, Philadelphia, said.
Police said they freed Sakinah from her chains and observed she was very thin and had sores covering her body. She was transferred to Albert Einstein Medical Center where she was being held and evaluated.
Sakinah's sister, Alissia Robinson, said she had no idea Bennett was allegedly keeping the woman, who suffered from schizophrenia and has the mental capacity of a 10-year-old, restrained.
"I don't want my sister to go through this again and she's never going to go through this again," Robinson said, adding that she plans to take Sakinah into her home.
Bennett was charged with terroristic threats, simple assault, public drunkenness, aggravated assault, unlawful restraint, false imprisonment and related offenses, police said.