Sarty's sisters are twins. We don't know how old they are. One of them seems to be named "Net," but Net might also be Sarty's aunt's name. It's not clear. In any case, Sarty doesn't seem to like them at all, and as far as we can tell, he hardly interacts with them. The narrator describes them as big, lazy, and selfish girls. This seems to be from Sarty's perspective.
We notice that their mother seems to be trying to shield them from unpleasantness. When Abner makes them clean the rug, she tries to take over the task. She doesn't seem to require them to help her with the cooking and chores, at least according to Sarty. Abner is the one who makes them help. This completely irritates Sarty. That he isn't able to muster any pity or empathy for his sisters might suggest that they have done something to hurt him. Or, it might be further evidence of the deep fissures Abner's tyranny creates among the members of his family. What do you think life is like for Sarty's sisters? Since we aren't given much in the way of concrete character details, we can't do much but speculate about these girls.