Either Kuber or Rambha (Swarg ki apsara) cursed Ravan.. By the way, here is the story I found on Sita being Ravan and Mandodari's daughter. Vedavati (Kushdwaj's daughter) was devotee of Vishnu and wanted to marry only Vishnu. She is described to be a very beautiful woman. Ravan sees her and proposes to marry her but she rejects. He rapes Vedavati. Vedavati immolates herself on a pyre and vows to return adn be the cause of Ravan's death. In the meantime, she receives boon to be married to Vishnu's human avatar. Ravana and his wife Mandodari grow estranged from each other. Mandodari soon finds herself pregnant, and fears that the child within her could be the reason of her husband's death, as per Vadavati's oath. Mandodari goest to her her father's home in Jodhpur, and then on a pilgrimage, in order to prevent Ravana or anybody else to know that she is pregnant. As her confinement grows near, Mandodari seeks around for a suitable foster-home for her child. She discovers that Janaka is childless. A female child is born to Mandodari. Soon afterwards, just before Janaka begins ploughing a field preparatory to the intended rituals, Mandodari manages to spirit her baby into the field and into Janaka's path. King Janaka adopts her. sita came from earth and ended in earth. These legends build on ancient Indian traditions which hold, in wry spirit, that one's worst enemies are re-born as one's own children to fulfill the karma of one's sins. GOD KNOWS..