My brother girlfriend took the kids to another state is that legal? The are not married.
The kids have his last name
Depends on the facts and circumstances of the situation. For instance, is there a parenting plan in place that has been approved by a family law court? You should consider meeting with a family law attorney to discuss your case in more detail.
If they are not married, and he has not established his paternity of the children through the courts, then the mother is considered to have sole custody of the children. He needs to meet with a family law attorney as soon as possible to discuss his options.