Legal Question in Criminal Law in California

That joint account is HIS asset as well as yours, and it can be levied upon with a judgment.

You are not liable for your uncle's actions merely because you and he share a bank account. That means your own assets should be safe. But as Mr. Nelson notes, the joint account is simultaneously his asset and yours. A successful plaintiff could levy on the entire amount.

Your question suggests that there is no judgment against your uncle at the moment, or even any lawsuit that you know of. If you are really concerned about this risk, perhaps now would be a good time to withdraw your share of the funds and put them elsewhere. If you wait until after a lawsuit is filed, you may later have to defend your actions against an accusation that you are merely pretending your uncle's money is your own. Acting now should prevent that from happening if he is sued later.