How should the questioner act towards his 24-year-old drug-addicted brother, who wishes to sell his inherited share of land to spend on drugs, refuses to work or seek treatment, causes family problems, and does not respond to any advice?

You should continue to advise and pray for your brother who is addicted to drugs, because he is a safih (prodigal/spendthrift) who should be interdicted for his own good. A safih is an adult of sound mind who is incapable of managing his own money, spending it on forbidden things or on things that bring no benefit. The interdiction of a safih is by judicial ruling, according to the majority of scholars. If judicial interdiction is not possible, or if it would lead to a greater harm, then he must be persuaded to preserve or invest his money, while informing him that he will not be provided for if he squanders his money. If he insists on taking his money, then he should be given it, and there is no sin in that.