Will my sister's husband be sinful if he returns to working in banks due to the unavailability of other work, knowing that he refused to work in insurance companies for the same reason?

It is not permissible to work in an interest-based bank, as it involves assisting in sin and participating in usury, which exposes its practitioner to the curse mentioned in the noble Hadith. Your sister's husband has done well by leaving this job, and he should be patient and seek his reward from Allah, for Allah is with the patient and does not let the reward of the doers of good go to waste. And whoever fears Allah, He will open doors of provision for him, and the treasures of Allah are full. Allah tests His servant to expiate his sins, raise his rank, and bring forth from him stations of servitude like patience and supplication. He should also bear in mind the punishment for sin, the ill effects of disobedience, and the danger of unlawful wealth.