As per a story, Sita is believed to be daughter of Ravana and his wife Mandodari. Soon after her birth, there was prediction that she will be the reason for the destruction of Ravana lineage. Scared, Ravana scared decided to abandon the child. Sita was kept in a basket and buried in the ground where she was later discovered by King Janaka who adopted her.
This story appears in Sanghadasa’s Jaina version of Ramayana and also in Adbhuta Ramayana.
Reincarnation of Manivati
Another Story tells that she was incarnation of Manivati. According to Gunabhadra’s Uttara Purana, Ravana disturbs asceticism of Manivati and she pledges to take revenge on Ravana. She is later reborn as the daughter of Ravana and Mandodari. But, astrologers predict destruction of Ravana because of this child. So, Ravana orders to kill the child. Manivati is placed in a casket and buried in the ground of Mithila where she is discovered by some of the farmers of the kingdom. Then Janka, king of that state adopts her.