What is the Islamic legal ruling concerning money that you know belongs to your brother, being the price of land he sold that was not rightfully his, and he spends the money on drugs and alcohol, and you do not wish to give it to him?

It is not permissible to take the money of an imbecile (safih) who spends it on alcohol and drugs, nor to conceal it. Rather, it must be returned to its owner, or the matter must be brought before the judiciary for an interdiction to be placed upon him, because an interdiction on an imbecile can only be made by a judge's ruling.