My father has two sons and I am the younger one. My father shares a business with his partner who has one son. Our family are not practicing Muslims. As a matter of fact, my father and his partner have accounts in non-Islamic bank that deals with usury with its consumers. I asked my father to shift his account to an Islamic bank and once he talked to his partner, but they were not earnest enough to make changes. Now I am anxious about my future. The son of my father's partner and my elder brother are not fair enough to agree with me with that end in view. If I ask them and they are not motivated, then will it be lawful for me to share business with them? They will invest money mixed with usury from a non-Islamic bank whereas I shall deal from an Islamic bank. Shall I also be guilty in such case?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

Depositing money in a usury-taking bank is not permitted whether the depositor takes usury or not, since it is a cooperation in sin. Allah Says (what means): {… but do not help one another in sin and transgression…} [Quran 5: 2]

Besides, Jaabir narrated that the Prophet   said “Allah has cursed the one who consumes Riba (i.e. usury or interest), the one who gives it to others, the one who records it and the one who witnesses it (its transaction).”   The Prophet said: “All of them are equal in sin.” [Muslim]

Therefore, your father and his partner should repent to Allah and transfer their account to an Islamic bank from a usury-taking bank.

You should not give up advising them, guiding them and making them aware of the dangers of Riba in a gentle and soft manners.

It is forbidden for you to be a partner in this company if the funds of the company are being deposited in a usurious bank. If your partners do not deposit the amount of this company in usury-taking bank, then you may invest with them even if their personal money is deposited in usurious banks.

However, it is better to avoid dealing even in permitted matters with those who engage in usury as confirmed by the jurists.

We ask Allah to make your matters and ours easy, and grant all of us lawful earnings.

Allah knows best.