All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
Borrowing money with interest is forbidden and whoever does so without a necessity is obliged to repent to Allaah. Also, whoever borrows some money should return the same amount. Therefore, you should give back the same amount to your mother. If the bank remitted your mother’s debt, then this does not make you exempted from paying back your debt to her if she asks you to repay it, but you are obliged to repay the debt without paying any interests.
It should be noted that if the contract of getting the credit cards conditions that an interest be paid if there is a delay in repayment, then it is not permissible to deal with such credit cards. For more benefit, please refer to Fatwa 108346.
Allaah Knows best.