Ref. No. 40/866
In the name of Allah the most Gracious the most Merciful
The answer to your question is as follows:
It is not allowed to do job in interest based banks. If your brother has no other sources of halal income, you should avoid eating at his home. Now, it is your duty to help him find another halal job. It will be a great help from your side.
And Allah knows best
Darul Uloom Waqf Deoband