Praise be to Allah.
If your brother is in debt, and he has a salary which is not sufficient for him to live on and pay this debt, and he is someone who prays and will not waste the money in things that are not pleasing to Allaah, then there is nothing wrong with giving him some of the zakaah from your money.
Shaykh Sa’d al-Humayd.
Because he comes under those whom Allaah describes as (interpretation of the meaning):
“… those in debt (al-ghaarimeen)…”[al-Tawbah 9:60]
But if his debt is the result of something haraam such as ribaa or gambling, then do not give him anything until he repents to Allaah.