Is an aunt entitled to take possession of her two brothers' money after their death, or must it be divided among the heirs, given that they had authorized her to manage their money during their lifetime?

The agency is terminated by the death of either party, whether the agent or the principal. Your aunt was an agent for her two brothers, and one of them died. Her agency to dispose of his property became void, and the property became an inheritance that must be divided among the heirs. Your aunt is not entitled to take her brothers' money.