Am I obligated to repay the debt to my aunt's heirs, knowing that she exempted me from it if she died before its repayment, and she has indeed passed away?

What the aunt said is considered a discharge of debt conditional on her death, and it is permissible as a will. If the amount of the debt does not exceed one-third of her estate, then you owe nothing to the heirs. However, if it exceeds that, the matter depends on the heirs' approval. If they permit the increase, it becomes yours; otherwise, you must return it to them.