Does sin befall me if my mother repays me the amount of the bank loan I took out for her, in addition to the interest, or should I just take the original amount without the interest to avoid sin?

Borrowing with interest (riba) is forbidden and not permissible unless there is a necessity that cannot be averted except through it. The definition of necessity is that which is highly likely to cause a person to fall into perdition, or unbearable hardship, or prevent the attainment of the minimum standard of living for the poor. Necessity is assessed in proportion to its extent. A mother's demand for usury does not make it permissible, for there is no obedience to a creature in disobedience to the Creator. The borrower is responsible for the debt, and it is not permissible to be a means to what is forbidden. One must not borrow with usury, and if a usurious loan has been taken, it is not permissible to take the interest.