What is the ruling on the money the questioner's mother took from her brother (100 Egyptian pounds monthly), and what is the ruling on the bank interest the mother took, knowing that they were unaware of the prohibition of usury until years later?

If the money the mother gave to her uncle was a loan, then the monthly amount she received from him is usury (riba). If you were unaware of its prohibition, you are not obligated to give back what you took from the usurious interest, based on Allah's saying: ﴿So whoever receives an admonition from his Lord and stops, then he will have what has already passed, and his affair is to Allah.﴾ [Al-Baqarah: 275]. However, if you were aware of the ruling, then each of you must return the riba you received to the uncle along with repentance, based on Allah's saying: ﴿And if you repent, you will have your principal — you do not wrong, and you are not wronged.﴾ [Al-Baqarah: 279].

If the amount was given as Mudarabah (profit-sharing partnership), it is a فساد (corrupt) Mudarabah due to the fixed profit and guaranteed capital, and its reality is a loan. The uncle is obligated to return the capital plus the profit or minus the loss, and the uncle is entitled to a fair wage.

As for bank interest, whoever was aware of its prohibition should spend what they received from it on Muslim welfare, such as the poor and charities, and it should not be returned to the bank because it is difficult to identify its owners.