Would the money I receive be considered unlawful if my brother sold my apartment to an agent, and this agent sold it to someone whose money I am not sure is lawful?

The price you receive for your apartment is lawful and there is no harm in it, unless you know with certainty that it is unlawful. You are not required to investigate its source. Indeed, the Prophet, peace and blessings be upon him, used to deal with the Jews, who consumed unlawful earnings and usury. The basic principle is that a Muslim's wealth is lawful, and it is permissible to transact with it through buying and selling.