Suing for Custody

Hey so weird(ish) question here. My boyfriend and I have recently been talking a lot about if/ when we get married how we're probably going to sue his sister for custody of his nephew. His nephew is turning four this month and super behind developmentally due in large part to the fact that he doesn't play with other children at all, and his mother doesn't interact with him. Anytime a daycare provider mentions that he might need a learning support plan and that she needs to work with him at home she flips out and says that's their job. He just finished potty training because she didn't want to deal with the hassle of running him to the bathroom. He's on a smart phone all day every day so he doesn't bother her and she still just spends all day yelling at him for being bad and stupid, despite just being three. He also has multiple cavities as of the last time they went to the dentist because she only feeds him candy bars because she doesn't want to spend money/ cook. 
However what had me ready to call child services the other day was that she took him to work (on an 80 degree day) and left him in the car in a parking lot with a phone. It soon started to downpour too. She also has repeatedly slapped and shook his face and body out of frustration. 
I'm an early childhood education teacher, and I have worked as a nanny. And my boyfriend (his uncle) is not only the person leagally named who gets the child in the event of an emergency or death since his father isn't on the birth certificate. But my boyfriend also makes a good salary as a butcher. The child respects us. And goes to us in a crisis or excitement or to learn rather than his mom or her boyfriend. 
My question was, does anyone know what this process would be like in the state of Pennsylvania?