Examine the references to the boy's sisters in the story. How and why does Faulkner describe the sisters the way he does?

The first reference to Sarty's sisters occurs when Sarty and his father leave the store where the justice of peace has admitted that he can't rule against Abner but orders him to leave the area. After a scuffle with another boy, Sarty gets into the family's wagon, where

his two hulking sisters in their Sunday dresses and his mother and her sister in calico and sunbonnets were already in it, sitting on and among the sorry residue of the dozen and more movings . . .

The sisters are not described as being attractive or animated; they are part and parcel of the family's broken belongings.
When the Sartoris family reaches their next house, one sister remarks, "likely hit ain't fitten for hawgs," and both have to be prompted by their father to get down and help their mother unload the wagon. They do, and the narrator describes them as " big, bovine, in a flutter of cheap ribbons." Later that afternoon, when Sarty returns to the house, he knows his mother and aunt have been setting up the stove while the two sisters' "voices . . . emanated an incorrigible idle inertia." The sisters are characterized as lazy, ill-tempered, ill-mannered, and a drain on the family's meager resources.
When Abner orders the girls to set up the wash pot, one won't help carry the rug and tells the other to get the pot—but she (the other) orders Sarty to do it. The sisters are characterized as foul and not unlike livestock—Abner treats them in a manner similar to the family's mule as they ruin the de Spains' rug at his direction:

. . . the two sisters stooping over it with that profound and lethargic reluctance, while the father stood over them in turn, implacable and grim, driving them though never raising his voice again.

The last description of the sisters, who are revealed late in the story to be twins, is a parenthetical. It occurs as they fail to catch Sarty as he runs after his father and instead watch with "bovine interest":

(the sisters were twins, born at the same time, yet either of them now gave the impression of being, encompassing as much living meat and volume and weight as any other two of the family)

Faulkner consistently describes the sisters as cowlike. They contribute little to nothing to the family's well-being and exist as a contrast to the hard-working mother and aunt. They are as immoral and disinterested in life as Sarty is engaged and principled.