Should the heirs be liable for an alleged debt to the deceased's brothers, based on the statement of one of his wives, despite the lack of proof and their denial of her statement?

What your father's wife said and what your uncles are claiming is merely an assertion. They must provide lawful evidence to prove its validity, otherwise, it holds no value. For the burden of proof is on the claimant, and the oath is on the defendant. If your uncles prove their right with lawful evidence, then you must give them their share from the general inheritance before its division. However, if they do not prove it, then you must swear that you have no knowledge of any right they have over your father, and their claim against you will be dropped. What corresponds to the debt that your father's wife admitted to your uncles shall be taken from her share, provided she is legally competent to act.

Nevertheless, if you strongly suspect or are in doubt, it is better to pay your uncles what they demand with the intention of donation and charity, to preserve affection and kinship ties. If reconciliation is not achieved amicably, it is advisable to refer the matter to the Sharia court to resolve the dispute.