Am I obligated to repay part of a debt to my mother to her heir brother, knowing that my mother forgave me the debt before her death, and is it better for me to repay the amount to the heirs or to give it as charity on her behalf?

The testimony of the siblings regarding the forfeiture of the mother's right is considered valid and correct if they are upright, because filial piety does not prevent a son from testifying truthfully against his parents. The scholars have unanimously agreed on the permissibility of a brother testifying for his brother, unless he is suspected of bias in his favor. Therefore, your liability is cleared, and you are not required to pay anything to the heirs. As for donating on behalf of the mother and making the money a charity for her, this is an act of piety and kindness towards her after her death.