What should I do concerning my brother who has been committing abominations and grave sins for years despite possessing the qualities of a Muslim?

You must hasten to rescue your brother from the sin he has fallen into, remind him of Allah and His punishment, and explain the destructive consequences of sin in this world and the Hereafter, while being gentle and merciful in your address to him. Therefore, you must:

1. Guide him to righteous companionship, for a good companion benefits his friend. 2. Have some righteous people who hold a respected position in his eyes advise him. 3. Gift him some audio lectures and books that contain impactful sermons and reminders of Allah. 4. Supplicate for him in his absence that Allah may guide him and keep him away from the paths of misguidance.