What is the ruling on a brother boycotting his sister for 18 years over inheritance, and his refusal to meet her or reconcile with her despite her attempts and her forgiveness of him, and is she mistaken, and what are the consequences for her?

If the brother has wronged, transgressed, and oppressed because his sister demanded her right to inheritance, then he is obligated to submit to Allah's حكم (judgment) and be a helper to her. The sister is not the one who is negligent; rather, he is negligent regarding her right, and it is recommended to supplicate frequently for his forgiveness.