Is it permissible to give as charity the money I had borrowed from my deceased brother, given that his mother does not wish to receive it, or must it be given to his parents as an inheritance?

You must pay the debt to your brother's heirs, as it became their property after his death. If the heirs pardon you, they are entitled to dispose of it, so you may take it, or give it as charity on behalf of your brother, or on your own behalf. If his mother does not want her share of the money, verify with her whether she forgives you the amount or wishes it to be spent on good causes on behalf of her son. The same applies to his father. Then, you should act according to their response, if your brother has no other heirs. You are not permitted to donate this money as charity or dispose of it without the permission of the heirs, as the Prophet, peace and blessings be upon him, said: "A loaned item must be returned, a grant must be repaid, a debt must be settled, and a guarantor is liable."