In "The Apprenticeship of Duddy Kravitz," how is Duddy a parent to both his brother and his father?

Duddy is immediately fighting a battle he should not have to fight.  He is mostly forgotten or looked down upon by his father and uncle, who focus on the "successful brother," Lennie.  The grandfather is the only one who Duddy can find sympathy from and relate to.  Duddy is the more mature one, in some regards, because he is trying to get his father and family members to see that he deserves attention, as well, and that they should not focus on Lennie only.  Also, Duddy has been forced to grow up quickly because of the situations in his life and because of having little support, so he has experienced more at the age of 19 than most might experience in twice that age.