What is the ruling on a sister exploiting an old power of attorney to sell her brother’s land to herself without his knowledge, and what is the brother’s required course of action regarding this?

The agent must adhere to the terms of the agency and not exceed them. The sister's sale of the land without authorization from her brother is considered invalid and forbidden, and the land legally remains the property of her brother. She should be cautioned about the punishment for injustice and the heinousness of her action. The brother has the option to sue her or to forgo that for the sake of maintaining kinship ties.