Estate Law Questions? Ask an Estate Lawyer.
The insurance company is wrong. They are not in a position to declare that you are not your mother's heir because you did not pay the funeral bills. Also, they do not have any right to see the funeral bills. That is not relevant to your claim.
You would need to file complaint with your State's Insurance Commissioner against the insurance company for refusing to pay your claim. You can Google your State's Insurance Commissioner. Almost all State Insurance Commissioners have websites where you can file complaint online.
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