What happens when there was substantial property and maybe life insurance and no probate court was opened.

Hi my brother died back in 2018. At the time my brother, sister and I had no idea that our father did something that may have been illegal. My brother had a large life insurance policy and a fair amount of property. He had named our mother as beneficiary. However, my mother passed away before my brother. He never changed her as beneficiary. My father had some help getting the check from the life insurance company. There was also a 401 K retirement. This is how we learned that my father was supposed to open a probate court for my brother estate but he never did. He just sold his cars, furniture, clothes, everything. So the 401 k retirement is there. Now my father is trying to get me to sign a small estate paperwork and put nothing in the paperwork that was true, I am fighting them to sign it because it is incorrect and I believe my dad left us out of getting any amount of my brother ‘s estate. Now he has threatened me saying that he will get the 401 k money without my signature, like before, and not give me any of the 401 k. From all the long hours of reading into this. I believe my father committed insurance fraud and took all the money and property for himself. I am trying to get any paperwork that had my brothers information on his life insurance policy. But if I was not named in the policy, I am afraid that I will not be able to fight this at all. What can I do?