Who is Mercy from The Last of the Menu Girls and what is their importance?

A largely undeveloped character, Mercy is Rocio's younger sister. Growing up, they obviously spend a great deal of time together, although one wonders how close they truly are. When Rocio returns home after college graduation, she and Mercy argue a great deal over the condition of the family home, but Mercy is not inclined to clean it up. Mercy eventually marries, has children, and moves to northern New. Mexico.