Who is Sonia from Legends of the Fall and what is their importance?

Sonia is Nordstrom's daughter. As a child, she played tennis with father until she broke her ankle. Nordstrom promises Sonia a car after college graduation, and she has a number of friends who come to the Marblehead house. In the end, she does not want Nordstrom's money or the car he has promised her because she wants to find her own way in life.