Vickie Weston consented to the custody arrangement in 2002. That's not mentioned in the lawsuit filed at U.S. District Court in Philadelphia.
The mother's lawyer, Emeka Igwe, said Vickie Weston agreed to leave her daughter with Linda Ann Weston only on a temporary basis because she was struggling with the long-term effects of a brain aneurysm she suffered about 10 years prior.
Igwe said Vickie Weston attempted to regain custody of her daughter while city workers lied to her for years.
"They furthered the cover-up and the deception by giving her false reports," Igwe said. "I mean, they were actively telling her that her daughter was fine, that they were checking up on her, that she was going to school, when that wasn't the case at all."
Linda Ann Weston was convicted of third-degree murder in 1984 and served four years in prison for locking her sister's boyfriend in a closet and starving him to death.
After her release, police say, she enslaved four mentally disabled adults in an elaborate scheme to steal their benefits checks. The victims were found in October 2011 locked in her Tacony basement.
Prosecutors say one woman, whom Linda Ann Weston met at a party in 2002 and lured into captivity, died in her home in 2008 after years of being starved, drugged, and beaten. Linda Ann Weston is facing charges in federal court.
In 2011, Beatrice Weston filed a lawsuit against the city similar to her mother's. Igwe said that both lawsuits were now in federal court and that he expected them to be consolidated.
Igwe said Vickie Weston filed her own suit because she, too, was a victim.
"Nobody grieves like a parent when they find out their child has been harmed," he said.
Igwe said he anticipated financial damages could be "well into six figures," but stressed that number could change after further investigation.
In addition to the City of Philadelphia, the suit names Deputy City Solicitor Rick Ames, a DHS social worker, two lawyers assigned to work as advocates for the girl, and the nonprofit International Family Services.
DHS spokeswoman Alicia Taylor said she couldn't comment on pending litigation.
Beatrice is now 21. Igwe said she and her mother were in contact but not living together.