Assalamualaikum mufti sab
الجواب وبالله التوفيق
The most important weapon a mo‘min has is dua. You should make sincere dua for the betterment of the situation your brother is in. If your parents are alive request them to make duaas well as the biggest and most important wazifa is a parents’ dua for their children. Their dua is not hindered by anything and directly reach the arsh of Allah Subhanahu Wa Ta‘ala.
You should read the following 111 times after every Isha salah with punctuality, and blow on your younger brother, if he is not home blow it on his bed:
یَا شَاھِدْ یَا رَقِیْبُ یَا لَطِیْفُ
After every salah also read the following three times and make special duafor him:
واصلح لی فی ذریتی
واللہ اعلم بالصواب