The story of Mata Sita as daughter of Mandodari is found in the Adhbuta Ramayana – a Sanskrit version of Ramayan associated with the tantric Shakti cult. This text mentions that Sita was the daughter of Mandodari, wife of Ravana, the demon king.
As per Adhbuta Ramayana, Mandodari was a pious woman and she was unhappy with the demonic ways of Ravana. She was also saddened by the fact that Ravana showed no interest in her and neglected her.
Ravana had collected the blood of Rishis (holy saints) to perform some ritual. He had warned Mandodari not to touch it as it was poisonous.
One day fed up with the attitude of Ravana, Mandodari decided to end her life and drank the blood of Rishis believing it was poison.
She did not die but few days later she realized that she had become pregnant due to drinking the blood.
She then removed the undeveloped foetus and buried it in a paddy field.
The undeveloped foetus was protected and nurtured by mother earth. When it became a fully developed baby girl, king Janaka found her.
Thus Mata Sita had two mothers, Mandodari and Mother Earth. Her father is the spiritual power of Rishis.
Adhbuta Ramayana is a tantric Shakti text and Mata Sita is given importance. She is the all powerful Mother in it.