Q: If my brother left his weapon at the house in a rush and resulting in my partner getting charged for it.

A: It might. You would increase your likelihood of success if you got a criminal defense lawyer involved.

A: Affidavits are not admissible in court if the other side's lawyer (in this case, the prosecutor) objects. Prosecutor can object on the ground that the affidavit is "hearsay". Hearsay is any statement made outside of court.

It's possible that the prosecutor might not object to the introduction into evidence of an affidavit by your brother which says the weapon (I am assuming the weapon is a firearm) belongs to him because that, in and of itself, doesn't get your partner off the hook. If your partner knowingly allowed your brother to bring a firearm into your residence, he is in violation of the law. Your partner cannot physically be in any place where there is a firearm, nor can he allow anyone to bring a firearm into anyplace where he is.

Now if your brother brought the firearm into your place without your partner's knowledge and your partner did not become aware the firearm was there, then maybe your brother can testify to facts that would help your partner present a viable defense and get out of this mess. Live testimony by your brother is how you get around the hearsay problem of affidavits.

If the facts truly are that your partner didn't know the firearm was there, he needs a good criminal defense lawyer to present this defense effectively.

And if the "weapon " you're talking about here is NOT a firearm, then the above information should be completely disregarded and you should re-submit your question stating what the weapon is exactly.