There is an interesting story mentioned in some Hindu books and folk tales that mentions Sita as the daughter of Ravana. Legend has it that Vedavati, a pious woman, wanted to marry Srihari Vishnu. To marry Vishnu, she gave up worldly life and became a hermit and built an ashram on a riverbank.
Once while Vedavati was meditating on the riverbank, Ravana, the demon king, happened to see her. He was enamored by her beauty and tried to violate her. To escape from Ravan she jumped into a fire pit created for yajna. Before dying she cursed Ravan that in her next birth she will be responsible for his death.
In her next birth, Vedavati was born as Ravana’s daughter. Soon after she was born, a voice from the sky warned that the baby will be responsible for her father’s death. To save his life, Ravana threw the little girl child into the sea. The child fell on the lap of sea-goddess Varuni. Goddess Varuni took the girl child to the shores and gave her to earth-goddess Pirthvi.
Goddess Prithvi then gave her to King Janaka. As per Ramayana, Sita was found by King Janaka while ploughing a field.