There are many versions of Ramayana. The Indian Ramayana suggests Sita to be the daughter of King Janaka. As per Ramakian, the Thailand’s Ramayana, Sita was born to Ravana. In some other versions as well, Sita is depicted as daughter of Ravana.
So, the legend goes like this:
Vedavati, an incarnation of Goddess Laxmi, wanted to marry Lord Vishnu. To marry Vishnu, she gave up materialistic living and started following an ascetic life in a riverbank.
Once Vedavati was meditating on the riverbank, Ravana was passing by. Saw her ravishing beauty, he got enchanted and tried to molest her. To escape from Ravana, she jumped into a fire pit created for yajna.
Vedavati cursed Ravana that in her next birth she will be responsible for his death.
In her next birth, Vedavati was born to Ravana and Mandodari. Astrologers predicted ruin of Ravana because of this child. 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 Indian Ramayana, King Janaka found a girl child while ploughing a field. He braught up this child as her daughter Sita. Later, Sita became the cause of Ravana’s death.