What is the ruling of Islamic law and civil law regarding a brother seizing his mother's property and renting it out for a pittance without the consent of the other heirs, knowing that the mother suffers from Alzheimer's?

If the mother is afflicted with Alzheimer's, her financial transactions are not enforceable unless they are sound. If she is in good health, she is not permitted to favor some of her children over others without a legitimate reason, as the Prophet, peace and blessings be upon him, said: "Fear Allah and be just among your children." Therefore, what your brother did by manipulating her to make the contracts is not religiously acceptable.