Answer
(Fatwa: 65/65/M=1432)
According to shariah, the job of writing, reading and calculating interest based dealing is unlawful and a cause of curse. The income earned through it is not lawful and pure. If the job and income of your younger brother is of interest, you should avoid eating any thing given by him and convince him to adopt any halal and lawful job. If he feels regret and starts searching any halal and pure means of income by making tauba (repentance), there is no harm in living with him. Also, you should also help him with your precious advices.
Allah (Subhana Wa Ta’ala) knows Best
Darul Ifta,
Darul Uloom Deoband