The short answer is no. The question is not who owned the ammunition, but who knew it was there and had the ability to possess it if he wanted to? You might own the ammunition left at your brother's house, but if your brother knew it was present, knew where it was left and had the ability to possess it, he can be found in violation of the felon in possession of ammunition statute due to such constructive possession. Remember that the government (state and federal) has the burden of proof and they have to prove your brother guilty. It's best to consult with an attorney and discuss the particular facts of his case to know what defenses he might have to the charge.
It might help, but it really doesn't matter if it was yours. Your brother cannot be in possession, he doesn't have to actually own the ammunition or firearm.
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I agree with the other attorneys: the charge is not owning , it's possessing, and, more than one person can be charged with possessing the same item.
However, I think you should definitely discuss this with your brother's attorney.