Why "Ponnivala"?

Why do we call this story "The Legend of Ponnivala"?

Locally, this great oral epic is known by the names of a set of twin brothers, Ponnar and Shankar. But these two men are actually the eldest two sons in a set of triplets that includes a younger sister. The story is popularly known as “The Elder Brothers’ Story” (Annanmar Kathai, in Tamil).