Nick is in a constant battle with his body, dependent on technology and other people for his very life. He worked hard with his hands to earn and keep his place in America and so spurns those he feels use the this wonderful system for their needs rather than working for it. He is between life (and how he remembers it) and his immenent death. His son and his son's wife need help which he is helpless to give. He is angry in his impotence.
Tig has witnessed a great deal of life in her short one. She watched her parents maintain their love during adversity, her brother run full bore into a life without real joy, the basest poverty among people who are capable of seeing joy in adversity. I see TIg as a savior figure in this book. Her patience with Nick shows her understanding of his position in the family now. She actually teaches Willa (her mother) to recognize and enjoy life without wealth. She nurtures her grandfather, her nephew and her family (with her cooking). She sees "Dusty" as a sign of the future.
I love the ironic
Walt Whitman Bridge locale as a site for Willa's and Nick's discussion. The old bridge metaphor is obvious - crossing from one generation into another or meeting in the middle. The naming of the Walt Whitman Bridge was historically very controversial. As a Transcendentalist, Whitman was a controversial poet, loving self and all people and their persona, shocking a more conservative sector of America. Willa would approve the Walt Whitman name on the bridge - Nick would not.