It is said that the Gods were fed up of Ravan and the demons who had made the life of Rishis and the gods miserable. It was time for Vishnu to take an avatar and Ram’s birth was imminent.
King Dasharath of Ayodhya was married to Queen Kaushalya, but they could not produce an heir. Dasharath married Kaikeyi, but the same and same with his third wife Sumitra too. After all the efforts, when the King did not have an heir, he was suggested to perform a Yagna and seek divine intervention.
During the Yagna, the Lord appeared from the flames and offered a bowl of divine potion for the Queens to consume. The King distributed the same equally amongst his three Queens. Some versions say, that both Kaushalya and Kaikeyi loved the youngest Sumitra so much that they decided to give some portion from their share to her and that is why Sumitra had two sons as compared to the other queens.
King Dasharath was soon blessed with four sons, Rama to Kaushalya, Bharat to Kaikeyi and the twins, Lakshaman and Shatrughna to Sumitra. The epic goes on to mention that Ram was born on the ninth day of Chaitra month (which is celebrated as Ramnavmi), Bharat early next day, i.e. on the tenth day of Chaitra and Lakshaman and Shatrughan were born during the latter part of the tenth day, thus establishing the seniority of Ram. Lakshaman and Shatrughan though mentioned as twins, the epic does not delve in the details of the same, w.r.t. them being identical twins or not.
Some version go on to complicate the distribution of the divine portion as half to Kaushalya, out of the balance half, Sumitra gets a significant portion, and as an afterthought, Dasharath again distributed the remaining between Kaikeyi and Sumitra. This amounted to Sumitra consuming the potion twice. During the times of mythology, twins was a known fact, except that there was no explanation for the same. Thus such instances were created by the authors to explain and justify the concept of twins. This was necessary, as mythologies world across is replete with examples of trouble after the birth of twins, due to a proper understanding of the concept! (This is a subject by itself which we will discuss sometime in future).
The birth of mythological heroes has always been ‘different’ which is a precursor to the events that follow. Be it divine intervention or through ‘yagnas’ (sacrifices) especially in Indian mythology, the birth of a hero has always been different. The same can be seen in other mythologies too.