King Dasharatha of ayodhya had three beautiful and virtuous wives- kaushalya,kaikey,sumitra.

But they did not have any children. 

King Dashratha performed a yajna as advised by Guru Vashishtha,the royal priest.

Out of the yajna fire,a divine figure emerged with a bowl of prasad for the Queens. 

In due  course, the queens gave birth to four divine sons - Ram(the incarnation of lord vishnu) was born to the eldest queen Kaushalya, Bharat was born to kaikey and the twins Lakshman and shatrughn were born to the youngest queen sumitra.