Laws vary from state to state.
My soon to be ex-wife is working on getting CS from the bio father and things seem to be working out. Find out who the bio father is and try to get him to pay, use the state's social service for this. The gov doesn't like the kids to suffer financially in this situation so if the bio father can't be forced to pay, your brother will probably have to. For me, from a legal standpoint, I won't be my son's father anymore, but emotionally, I will. Even before I was in this situation, I never liked the idea of someone paying for someonelse's child.