Mother Is Trying To Make Me Sign Away Inheritance

I have been a dependent of my stepfather since I was 4 years old. He passed away of cancer from water contamination at a government facility. When he died (I was 18 at the time) there was no will that I had ever seen. Mother claims the will is missing and that there are 3 witnesses to the will (one who is also deceased, the others we are no longer in contact with).
In order to receive the government settlement money, my mother wants me and siblings to sign a statement saying we are not on the will and will receive none of this settlement. I absolutely will not sign this piece of paper without seeing the will that is claimed to be missing. He died in state of Oregon. Any advice on how to proceed?
I would also like to note that my mother has a history of reckless spending. She is in rampant credit card debt and has already spent any prior inheritance she received from my stepfather (on clothes, furniture, plastic surgery, etc). I understand this is her choice, but I would be devastated to see money that I am entitled to potentially go towards something like that.
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