She is the real daughter of Lord Ravana and Mandodari, but because of her birth, she was predicted to be the destroyer of the giant family. Akshayakumara or your oldest half-brother was forced to put your young-half-sister in a casket and float her on the water, causing her not to know until YOU who was her older half - (brother or sister) who knew the truth decided to tell the truth to Sita. so that once this is done, the war will end