Quick: name Billy's sisters.
Ha, ha! Trick question. You can't do it, because Billy never names them. Even his dad just refers to them as "the girls," as if they're some sort of feminine collective. In fact, they come across less as characters than as objects that let Billy show what a good heart he has. Every scene they're in has Billy performing some action upon them: yelling at them, giving them something, or pitying them. Check out the way he talks about them:
After all, I had three sisters and they giggled all the time, too. (5)
Hearing a whimper, I turned around. There in the doorway to the room stood my sisters. I could tell by the looks on their faces that they had been watching for some time. They looked pitiful standing there in their long white gowns. I felt sorry for them. (19.76)
In both these instances, Billy is using his sisters to feel better about his situation. In the first, he rationalizes the fact that town girls are laughing at him by thinking about his sisters to convince himself that girls just act that way. In the second, he pities his sisters to avoid thinking about the fact that his dog is dying gruesomely in his arms.
Look, all we're saying is that we suspect Billy really hopes that his mom's next baby is a boy.