What is the solution for dealing with a brother who is concerned with worldly matters and does not care about the affairs of the afterlife, and backbites people, knowing that he prays and fasts?

Being infatuated with worldly life and following whims are causes of heedlessness and hardness of heart. Backbiting and slandering people are dangerous sins, as Allah Almighty says: "And do not backbite each other. Would one of you like to eat the flesh of his brother when dead? You would detest it. And fear Allah; indeed, Allah is Accepting of repentance and Merciful." Continue to advise your brother gently and kindly, seizing appropriate opportunities, and you can seek help from some brothers and friends.