What is the way to deal with a brother who steals, threatens his parents, and physically assaults them, knowing that he does not pray or fast?

Your brother is committing grave sins, the greatest of which are abandoning prayer and disobeying parents. He must repent immediately. You can seek help from family members he respects to influence him, try to sit with him, and remind him of Allah and your rights. If this does not work, there is no harm in reporting him to an official authority to prevent him from assaulting others. Do not stop supplicating for his guidance. As for your mother's action of writing the apartment contract in your and your sister's names, thereby disinheriting your brother, please refer to Fatwa no. 59305 regarding its permissibility.