Sumali wanted Kaikesi to marry Visharva. Visharva is Pulstya's son, son of Brahma and Kuber is Visharva's son. Sumali wanted a heir who would get back Lanka from Kuber and he knew that the heir needed to be powerful than Kuber. So, he decided to send his daughter to Kuber's father to have a child. The child of Kaikesi and Visharva would be powerful than Kuber as he would also have blood from Rakshasa. Well.. this is what the serial showed. He had son older than Kaikesi and could not reject Kaikesi as he was a Aryan and they could not say No when somebody asked for something.. hahahaha Visharva already had a wife (Kuber's mother). Vishnu went to Kuber to ask him to stop his father from marrying Kaikesi but by the time he reached, they were already married. Kuber's mother came to Lanka with Kuber and never returned.