It was believed that during the Dwapara Yuga, the Yadavas were made up of two groups – the Vrishnis and the Andhakas.
The Yadavas was supposedly a republic and not a monarchy as it was in the other parts of the country.
In this republic the area of Mathura and thereabouts was ruled by the Andhakas. The ruler of the Andhakas was Ugrasena.
The Vrishnis were ruled by Surasena. Surasena's son was Vasudeva.
Ugrasena and Surasena were generally considered as the chieftains or the kings by their respective tribes.
Thus Vasudeva was one of the main noblemen of the Yadavas.
One day, he approached his friend Sini for some help.
Sini of the Yadavas was the best friend of Vasudeva and frowned wondering what the nobleman wanted.
"Princess Devaki, the daughter of King Ugrasena is having her swayamvara." Vasudeva said quietly. [Swayamvara is where a princess chooses her own husband.]
Sini understood immediately.
Sini realized that Devaki and Vasudeva loved each other and that he had to kidnap the princess during the wedding.
Sini happily agreed to this and successfully kidnapped the princess and brought her to Vasudeva.
Even the marriage of Devaki and Vasudeva was performed with great pomp and show.
At that time, no one paid attention to Kamsa, the son of King Ugrasena and Padmavathi. But at that time, no one had any reason to.
(There are four anecdotes connected with this incident.
One, is that Vasudeva also married the other daughters of Ugrasena. It remains unknown as to why Ugrasena conducted a swayamvara, if Devaki wanted to marry Vasudeva in the first place. And there is absolutely no doubt that Vasudeva loved Devaki a lot. Vasudeva had done a lot in his life to save Devaki's life.
One of the other wives of Vasudeva is Rohini. She is Devaki's co-wife and is very important in Krishna's story.
The second anecdote is about Sini. Sini fought off the king Somadatta during the kidnap of Princess Devaki.
This battle between Sini and Somadatta caused an almost generational feud between Sini and Somadatta which ended only in the Kurukshetra war where a descendant of Sini – Satyaki killed Somadatta.
The third anecdote is that Vasudeva was the brother Nanda. According to the book "Harivamsa", Nanda was the head of the Gopa tribe which was another powerful group among the Yadavas.
The fourth anecdote is that of Kamsa.
The Puranas mention Kamsa as the son of Ugrasena and Queen Padmavathi. But there have also been some off shoots regarding this story. Padmavathi is the daughter of King Satyaketu of Vidharbha. It is said that after her marriage, she had once gone to her father's home and there she was seen by a gandharva (Gandharvas are celestial singers and dancers in the Court of Indra) named Dramila or Godila.
Godila lusted after Padmavathi.
And so Godila took the appearance of Ugrasena and approached Padmavathi. Padmavathi presumed Godila to be her husband.
Their mating is said to have resulted in the birth of Kamsa.
Despite his parentage, it can be presumed that until Devaki's marriage, Kamsa loved his sister as any brother would and that Kamsa bore her no ill feelings towards her.)