Is merely disavowing usurious gains sufficient, or is it obligatory to donate an equivalent amount to charitable causes if the mother took it from her daughter and then gave her an amount from another source?

Investment certificates issued by usurious banks are forbidden loans, as they are bonds representing an obligation to pay their amount with an interest attributed to them. You did well by withdrawing your money.

There is no sin in your past use of the interest, due to your ignorance of its prohibition. The evidence for this is the Almighty's saying: "So whoever receives an admonition from his Lord and desists, he may have what has already passed." Therefore, there is no sin upon you regarding the amount your mother gave you.

It is necessary to ensure that the Islamic bank adheres to Sharia compliance in investing money, because some of them invest a large portion of the money in usurious bonds or treasury bills. If the bank in which you deposited the money does that, then promptly withdraw the money and put it in a current account if you need to keep it safe.