How can the forbidden interest mixed with lawful capital be calculated upon the death of a father who used to deposit his money and its interest in banks?

Depositing money in usurious banks and taking interest from it is one of the gravest sins. However, depositing money in them is permissible out of necessity to preserve funds if no other means are available. In such a case, it is not permissible to benefit from the usurious interest; rather, it should be taken and spent on general Muslim welfare and charitable causes. The wife's father must be advised to abstain from usury. If his wealth reaches you through inheritance, the heirs must dispose of the unlawful portion of it and spend it on Muslim welfare.