Lav and Kush returned with Ram, Lakshman, Bharat and the royal entourage to Valmiki's ashram. There Valmiki waited for them. He had known, through his powers, that Ram would come with his sons. Sita waited too, to see her sons safe with their father.
'Come back to Ayodhya with me, Sita, and take your rightful place as my wife and queen,' said Ram.
But Sita refused. 'My only wish was to see my sons safe with their father,' she said. 'I have now seen that happen. All that I now desire is to return to my mother, the Earth.' So saying, Sita folded her hands, and called upon her mother to take her within her. All at once the earth opened beneath Sita's feet, and before the astonished gaze of the King of Kosala and his court, Sita vanished from this world forever.
Ram was heartbroken. But he returned to Ayodhya with his sons Lav and Kush. Ram ruled Kosala for many years more, with Sita's golden image forever by his side. Lav and Kush were taught all the arts of kingship, and when they were old enough, Ram built two capital cities - the city of Shravasti for Lav and that of Kushavati for Kush. Ram then left his throne and his kingdom in the joint care of his twin sons. His days on earth were over, and it is said that very soon after he walked into the river Sarayu, to be welcomed by Brahma and to be absorbed into his being as Vishnu.
Lav and Kush, the Royal Twins
See also: Ram, Prince of Kosala