I'm Lucy, and I'd be happy to answer your questions today. I'm sorry to hear about your situation - it sounds heartbreaking for everyone involved.
Your mother can tell the judge that your brother has stated he has no intention whatsoever of following the court order, and the judge will consider that when entering a final order. She can also comment on what she sees when the child is with her and ask that the parents be ordered to attend counseling (separately) and/or that they be ordered to take your nephew. If your brother doesn't follow the custody order, he's in contempt of court. Also, he could be charged with custodial interference, which is a misdemeanor. He's allowed to move, but he needs to make sure the child makes it to any other places he is legally required to be - and a judge actually could put a restraining order to prevent him from moving. New York is a BIG state. The judge can tell him that he's not allowed to move to the opposite end of the state if doing so makes following the custody order impossible.
At seven years old, if your nephew can convince the judge that he understands the difference between the truth and a lie, and that he knows what it means to promise to tell the truth, he will be allowed to testify. The judge will be more interested in why your nephew prefers to live with your mother, but it sounds like he has very good reasons. The main problem is that the parents get preference over non-parents when it comes to custody. Weekend visits are one thing, but to get a full week out of every month, it could be difficult. Your mother will need to be prepared to present all of the evidence she has that it's best for the child to be with her right now. It may help to make a list of arguments, and what evidence applies to each, so she doesn't forget anything.
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