What is Rukmani’s family background? Why did she marry Nathan?

Rukmani grew up as the fourth and youngest daughter of a village headman. During her childhood years, her father had been a man of consequence and wealth.
As a result of their father's position, Rukmani's two eldest sisters had managed to make good matches; they married well, with the requisite dowries, jewelries, and lavish wedding ceremonies expected of daughters of a village headman. By the time her third sister married, however, Rukmani discovered that her father's influence had been infinitely diminished. It was now the tax collectors who held all the power and consequence in the villages.
As a result of this change in affairs, Thangam, Rukmani's third sister, only managed to secure a modest wedding and to wed with minimal jewelry on her person. By the time Rukmani was to marry, the family could not afford a dowry at all. Without beauty and without a dowry of any sort, Rukmani was forced to marry an impoverished tenant farmer at the age of twelve. The family's poverty and lack of consequence meant that they could never hope to make a good match for Rukmani, even if they wanted to. This is why Rukmani ended up being married to Nathan.