Track a coin

I had I very rare coin. I don't want to say which because it is so rare. I feel people might focus on that instead of my question. Which is, can I back track the chain of custody of said coin. The value of which was estimated well over 6 figures by the auction site just before the auction. To show that my alcoholic, coke headed brother had sold it. I know I can't do anything about it. It's more to prove I'm not a liar.