What is the ruling on accepting a gift from a mother who deposits her money in usurious banks and believes that this is permissible, given that there is a fatwa by Ibn Taymiyyah that permits dealing with someone whose money is unlawful if they believe it to be lawful?

If the mother invested her money in usurious banks believing it to be permissible based on a reliable fatwa, then there is no sin upon her. However, if she was seeking dispensations and following her desires, then she is sinful. There is no sin upon the children in benefiting from this money in times of poverty or ease, as it is mixed money.

As for her not paying Zakat on the money, this is a calamity, and Zakat al-Fitr does not suffice for it. She must repent and pay the Zakat on her money for the past years to avoid the severe punishment mentioned in His Almighty’s saying: "The Day it will be heated in the fire of Hell and therewith branded will be their foreheads, their flanks, and their backs, [it will be said], 'This is what you hoarded for yourselves, so taste what you used to hoard.'" And in the noble Hadith narrated by Abu Hurayrah.

The mother must be admonished in various ways to repent, pay the Zakat on her money, and cease usury, while clarifying the possibility of her investing in Islamic banks where the profit is lawful, and warning her against prohibited usury as in His Almighty’s saying: "Those who consume interest will not stand [on the Day of Resurrection] except as one stands who is being beaten by Satan into insanity. That is because they say, 'Trade is [just] like interest.' But Allah has permitted trade and forbidden interest. So whoever has received an admonition from his Lord and desists may have what is past, and his affair rests with Allah. But whoever returns [to usury] - those are the companions of the Fire; they will abide eternally therein."